I've come across a game-ruining problem that seems to have started today, and I have no idea why it's doing it. Whenever a lemming walks into an interactive object (Level exit, lava, trap etc), it'll simply ignore it and act as if it wasn't there (So if you have lava at the bottom of the level, the lemming will fall through it and die because of falling through the bottomless gap, not because of the lava). It also means I cannot complete levels, as the lemmings do not react to the exit.
Does anyone know why it's happening, and could explain how to stop it? All help appreciated.