It might be that one or more of the RAM sticks themselves got fritzed. I've had problems with defect RAM before, and the main symptom of this was random freezing and bluescreening. Removing the offending stick(s) got rid of the problem entirely, so if you can, it's worth investigating. That said, the freezing would usually happen well before the 12 hour mark (which is a LONG time of non-stop usage), but i guess it's always worth testing out all possibilities??

Also, i can't remember exactly how they look inside, but if you're on Windows, try looking for MEMORY.DMP inside the Windows folder, it MIGHT provide clues (... honestly i'm not sure, but hey!).