Occasionally, for no particular reason that we've ever been able to work out, the savegame corrupts. There's no consisetncy in it and it can happen anywhere and at any time. For example, in a game many years ago, my party was in a corner of the caves under Castle Kriegspire, and I turned them clockwise to leave it. The game crashed to the desktop. I had saved just shortly before, so I restored the position, went to the same corner, and tried it again. Same result. So I again restored the position, went to the same corner, and turned them anticlockwise to get out of the corner. No problem, the game worked perfectly. It just makes no sense at all.
If you don't have a backup savegame, try reloading the autosave. You might get lucky. Best wishes, anyway.