I had the same problem with my custom built PC, its pretty high end but that still doesnt stop things from going wrong! lol
All i did was open it up and check every signle connection because it could be ANYTHING! It might not and knowing sods law will not be to do with the graphics card.
Its possible that you fried something when you opened up the computer, its quite uncommon but you can fry parts like graphics cards/sound cards/motherboards just by touching them (you probably know this anyway) but is possible thats what happened. Do you notice anything else? I once fried my motherboard on my old machine (luckily just a £40 board!!) and when teh computer booted there was no beep, although not all computers beep and alot of home built machines dont have the little electric speaker that produces it.
Anyway just check EVERY LITTLE THING!!! Problems and solutions are usualy completely ramdom with computers. A loose cd-drive SATA cable once stopped me from logging into MSN messenger!! I know a fair bit about computers but alot of issues still amke no sense to me! I fixed my monitor problem by checking everything was plugged in right (even fans!)