New threat.

I tried loading the game by replacing the map in \maps\ with a new one (hard). I re-killed all the zombies – the result is the same. Do I have to start all over again???

If it crashes to the desktop, the application closes itself, but when it freezes, it freezes completely, and no task manager helps (even if you dance around with a drum).
That's the problem: I don't know where the game saves and where it doesn't – besides the battles, which you ideally want to complete with minimal losses, you also encounter other surprises.
For such maps, you should also specify the minimum system requirements in the description. Although it's strange – my computer is newer than the game; for example, Skyrim runs on it at maximum graphics settings.