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From the game perspective, the situation you're describing is Off the Rails (also known as Plot Derailment, which is very close to the "derailed plot" in your post title). That's regardless of how "easily" things went off the rails.
In-story, there may also be elements of You Can't Thwart Stage One as a Subverted Trope (although it sounds like they scarcely reached "stage one").
Also, question — why was the Big Bad attacking the hero party apparently alone and head-on on their first confrontation? Talk about Suicidal Overconfidence. He at least should have used Mooks to scope out the heroes' capabilities and realize, "Oh, they've got vorpal swords. I almost forgot those are a thing. I'm buying neck armor, pronto." Either that, or introduce the Backup Twin or Suspiciously Similar Substitute, or that the wererat they killed was Actually a Doombot or Body Double. So many ways out of this.
Edited by KD
^^ I don't know why it was attacking the party. The post was pretty vague, and was in like 2002 so I might not even be remembering it right. That said, I agree that it's really dumb he didn't do one of those tropes you listed as a solution. Hell, he could even make the encounter be All Just a Dream (maybe making it a prophetic dream if he felt just having it be a normal dream was too much of a copout. Even that would be better than just "you derailed my campaign and I can't think of anything, so we're done."
This is a Tabletop Game trope but that's not an option. Anyway, what's the trope for when the Gm sets up some elaborate plot for an adventure (or worse, a whole campaign) that will completely derail if the players do something, and doesn't bother to come up with a plan B in case that happens? For example, I remember on the old Dungeons & Dragons forums there was some thread about bad D Ms stories. One was something about an epic level adventure where they were going to some dungeon that was so scary even the gods were afraid to go in. Early on they get attacked by a wererat, and fight back. One of the characters has a vorpal sword and gets a critical hit, cutting the thing's head off, and it dies. The DM had a fit that they ruined his adventure and it was over then and there. They never found out what would have happened, but it seems pretty obvious that wererat was going to secretly be the Big Bad (or someone else important), and it was meant to be Hopeless Boss Fight but they beat it anyway.
Edited by Bootlebat