Several people have complained about this and it just happened to me, just lost 2 hours of progress.
Sometimes the autosave stops working. There's a way to tell if it's happening.
Press start, you will bring up the menu. If it says "less than X minutes" ago you're fine. If it's stuck on "autosaving, please wait" you're screwed and have to exit the game. It wont let you exit to the menu and if you try to exit with the PS button it will probably turn off your console but it's the only way.
SO HIT START REGULARLY and check if the autosave is updating.
God I'm so pissed, I feel like breaking something. 2 ******* hours!