Do you have back up files ? If yes then uninstall your game and re-install WITHOUT THE LAST CHECKPOINT ( meaning delete the last checkpoint from back ups ) . You might have to do a couple of missions or something but you wanna play right? So go for it.
If you don't have back ups then maybe there will be save game files in your APP.CACHE ( Appdata ) folder. So go there and delete the last save and restart the game.
If this doesn't work then all you can do is uninstall the game and delete all the files and replay the whole shit.
This would probably work. Hope it helps.
P.S - I'm not good at stuff on PC like steam etc so I cannot be sure ( forgive me ) . I'm a console-only gamer =D