Have you tried a starting new game rather than loading a previous saved game?
he cannot launch the game, how is he supposed to start new game or load save file???
OP: have you installed/uninstalled or changed any software or settings on your PC before this problem presented itself??
Also, 36hrs game, I will guess that you haven't put any mods yet, but if you have try uninstalling/disabling them.
Also, while steam verifies integrity, this game being modable it doesn't to a good job at it. The best way to try would be to copy your save files elsewhere and reinstall the game from scratch (if bandwidth is not a problem for you)