SAN FRANCISCO -- Lighting has been restored and play has resumed at Candlestick Park following a second power outage interrupting the game between Pittsburgh and San Francisco.
I've copied and pasted Rule 17 from the NFL rulebook covering what would happen in the case of game cancellations:
Emergencies, Unfair Acts
NON-PLAYER ON FIELD
If any non-player, including photographers, reporters, employees, police or spectators, enters the field of play or end zones, and in the judgment of an official said party or parties interfere with the play, the Referee, after consulting his crew (13-1-7 and 15-1-6), shall enforce any such penalty or score as the interference warrants.
If spectators enter the field and/or interfere with the progress of the game in such a manner that in the opinion of the Referee the game cannot continue, he shall declare time out. In such a case he shall record the number of the down, distance to be gained, and position of ball on field. He shall also secure from the line judge the playing time remaining and record it. He shall then order the home club through its management to have the field cleared, and when it is cleared and order restored and the safety of the spectators, players and officials is assured to the satisfaction of the Referee, the game must continue even if it is necessary to use lights.
If the game must be called due to a state or municipal law, or by darkness if no lights are available, an immediate report shall be made to the Commissioner by the home club, visiting club and officials. On receipt of all reports the Commissioner shall make a decision which will be final.
The NFL affirms the position that in most circumstances all regular-season and postseason games should be played to their conclusion. If, in the opinion of appropriate League authorities, it is impossible to begin or continue a game due to an emergency, or a game is deemed to be imminently threatened by any such emergency (e.g., severely inclement weather, lightning, flooding, power failure), the following procedures (Articles 5 through 11) will serve as guidelines for the Commissioner and/or his duly appointed representatives. The Commissioner has the authority to review the circumstances of each emergency and to adjust the following procedures in whatever manner he deems appropriate. If, in the Commissioner’s opinion, it is reasonable to project that the resumption of an interrupted game would not change its ultimate result or adversely affect any other inter-team competitive issue, he is empowered to terminate the game.
The League employees vested with the authority to define emergencies under these procedures are the Commissioner, designated representatives from his League office staff, and the game Referee. In those instances where neither the Commissioner nor his designated representative is in attendance at a game, the Referee will have sole authority; provided, however, that if a Referee delays the beginning of or interrupts a game for a significant period of time due to an emergency, he must make every effort to
contact the Commissioner or the Commissioner’s designated representative for consultation. In all cases of significant delay, the League authorities will consult with the management of the participating clubs and will attempt to obtain appropriate information from outside sources, if applicable (e.g., weather bureau, police).
If, because of an emergency, a regular-season or postseason game is not started at its scheduled time and cannot be played at any later time that same day, the game nevertheless must be played on a subsequent date to be determined by the Commissioner.
If there is deemed to be a threat of an emergency that may occur during the playing of a game (e.g., an incoming tropical storm), the starting time of such game will not be moved to an earlier time unless there is clearly sufficient time to make an orderly change.
If, under emergency circumstances, an interrupted regular-season or post-season game cannot be completed on the same day, such game will be rescheduled by the Commissioner and resumed at that point.
ALTERNATE DATES, SITES
In instances under these emergency procedures which require the Commissioner to reschedule a regular-season game, he will make every effort to set the game for no later than two days after its originally scheduled date, and he will attempt to schedule the game at its original site. If unable to do so, he will schedule it at the nearest available facility. If it is impossible to schedule the game within two days after its original date, the Commissioner will attempt to schedule it on the Tuesday of the next calendar week in which the two involved clubs play other clubs (or each other). Further, the Commissioner will keep in mind the potential for competitive inequities if one or both of the involved clubs has already been scheduled for a game following the Tuesday of that week (e.g., Thanksgiving).
If an emergency interrupts a postseason game and such game cannot be resumed on that same date, the Commissioner will make every effort to arrange for its completion as soon as possible. If unable to schedule the game at the same site, he will select an appropriate alternate site. He will terminate the game short of completion only if in his judgment the continuation of the game would not be normally expected to alter the ultimate result.
In all instances where a game is resumed after interruption, either on the same date or a subsequent date, the resumption will begin at the point at which the game was interrupted. At the time of interruption, the Referee will call time out and he will make a record of the following: the team possessing the ball, the direction in which its offense was headed, position of the ball on the field, down, distance, period, time remaining in the period, and any other pertinent information required for an efficient and equitable resumption of play.
Extraordinarily Unfair Acts
The Commissioner has the sole authority to investigate and take appropriate disciplinary and/or corrective measures if any club action, non-participant interference, or calamity occurs in an NFL game which he deems so extraordinarily unfair or outside the accepted tactics encountered in professional football that such action has a major effect on the result of the game.
NO CLUB PROTESTS
The authority and measures provided for in this entire Section 2 do not constitute a protest machinery for NFL clubs to avail themselves of in the event a dispute arises over the result of a game. The investigation called for in this Section 2 will be conducted solely on the Commissioner’s initiative to review an act or occurrence that he deems so extraordinary or unfair that the result of the game in question would be inequitable to one of the participating teams. The Commissioner will not apply his authority in cases of complaints by clubs concerning judgmental errors or routine errors of omission by game officials. Games involving such complaints will continue to stand as completed.
PENALTIES FOR UNFAIR ACTS
The Commissioner’s powers under this Section 2 include the imposition of monetary fines and draft-choice forfeitures, suspension of persons involved in unfair acts, and, if appropriate, the reversal of a game’s result or the rescheduling of a game, either from the beginning or from the point at which the extraordinary act occurred. In the event of rescheduling a game, the Commissioner will be guided by the procedures specified in Rule 17, Section 1, Articles 5 through 11, above. In all cases, the Commissioner will conduct a full investigation, including the opportunity for hearings, use of game videotape, and any other procedure he deems appropriate.
Lots to digest there. Hopefully, the game continues without incident.