Pipe for Puzzle lost, went through floor.

[Adny]

Try reloading an even earlier save file; it will most likely land you in one of the adjacent levels (i.e. the morgue, cistern, or cistern entrance) but it will work after that. Like I said, Frictional Games made sure it autosaves every time you enter a level, and I'm pretty sure these saves don't overlap, so you should be good to go.