No video signal.

Hi guys, I have a problem hopefully you guys can help me out.
I just put a new computer together and i was working fine for 3 weeks then i came home one day and the computer was on but no video signal. I reset the computer and got no video from it. I replaced the motherboard because I thought it died on me but still the same problem, i took everything apart and put it back together again and still the same problem. All the fans are working the video card works because i tested it on another computer, i also tested it on another monitor that i know works.. still no luck. When I listen for the POST beeps I get a long continuous beep. Manual says the graphic card is not installed properly.. but it is, I mean it worked before..... Please help me!

Specs:

CPU: Athlon 64 x2 7750 kuma 2.7ghz
MOBO: Gigabyte GA-MA770-UD3
RAM: OCZ Reaper HPC 4GB
PSU: 550watts BFG modular psu
Hard Drive: Velociraptor

I dont think the rest matters because I am not hooking them up until i get video. Right now i have 1 stick of ram, cpu , mobo, and video card connected. no video though output though...