You got a corrupted savegame. This is fixable but requires some knowledge manipulating game data. I recently fixed one: information concerning a module was missing fom the saved game and I extracted the info from the original module and inserted it in Savegame.sav - triggers were a little weird but I was able to continue.
Easiest way to go around this remains reloading a previous save game
Keep more than one save game when you reload.