I've come to learn after a while that it could be any plethora of things and just to prepare for the worst. Try the easiest/cheapest first but be prepared for something more ugly.
I had an older Dell laptop of classic '1.5 inches thick closed, 700Mhz P3 era' that eventually died with POST freezing and I think I chalked it up to the southbridge going on the fritz as it stopped recognizing both the cd-rom and the internal hard disk despite attempts to correct it. Given it was not usb bootable it was essentially dead at that point.
Computer's are finicky creatures sometimes. --
I swear, some people should have pace-makers installed to free up the resources. Breathing and heart beat taxes their whole system, all of their brain cells wasted on life support.-two bit brains, and the second bit is wasted on parity! ~head_spaz