My girlfriend's parents' computer randomly shut down last night and rebooted to an error - "Boot disk missing please insert system disk". Restarting the computer did nothing, but shutting down the computer and leaving it overnight fixed the problem. Apparently this has happened once before, and the same fix of just not starting the computer for several hours fixed the problem. CHKDSK is running right now on it, but I don't think it's found anything yet. Any idea what might be causing this problem? My guess is heat or a dying hard drive, but I'm not sure.