[Garet]

Crap, I forgot to mention this:

Do not assume that cables are good (tight, no damage), or that vid cards are seated properly. ;-) If you have opened your case or move your computer occasionally, there is the chance that you have jiggled something loose.

There is also the chance that a mouse (or some other dastardly vermin, like a kitten) decided that your cable needed some 'modifications'. I once had a kitten that worked very hard to ensure that any phone line in the house was good and dead. The bastard. :-)