"Oh, apparently we're going post-apocalyptic."
Different GMs will handle this sort of scenario differently. Some may despair, some may fault the player, some may retcon away the issue, which I sometimes do myself.
However, I do prefer rolling with the punches. Either by being aware of what's going on (or faking it by having an NPC i.e. holding the detonator on the other side of the room), or by accepting the event and letting the player(s) deal with the (potentially catastrophic) consequences.
For instance, back when I played Vampire the Masquerade, one scene had the (vampire) player characters encountering a very powerful vampire in torpor, a sort of hibernation/coma. Weaker vampires, in that game, can cannibalize stronger vampires for power. This is very much considered murder and in this case, the sleeping vampire was "on their team".
Of course, one of my players decided to "go for it". So he did, and the rest of the campaign was the group running from other vampires and their catspaws. Where that vampire "team" or their mortal servants couldn't get to the characters, they gave the characters' location to Vampire Hunters or Werewolves.
And as the finale, the vampire "father" of the sleeping vampire finally cornered them, promised them he wouldn't kill them if they surrendered, then forced them into hibernation and buried them, alive, at the deepest level of Yucca Mountain.
"I am a man of my word; you will not die now, nor for a long, long time to come. Perhaps, in that time, you may spend some of it thinking of what might have been."