My guess is that it's a connection problem; inspection all connectors and be sure no pins are bended or damage then reconnect securely. let us know how it goes, Good luck.
I installed a new video card a while ago (a year ago probs) and everything all worked fine. Recently I unplugged my computer and moved it to paint my room. When i put it back i plugged the monitor cable into the video card and plugged everything else in and turned it on to get a no signal appearing on the screen. I thought to myself AH this again - i had this problem before and i simply went into device manager (with the monitor plugged into the onboard graphics) and click disable on the on board graphics. It would then ask me to restart my computer - i would do so and take the cable out of the on board and put it in the dedicated graphics card (8600gt), and it would usually work.
I tried this but to no prevail. I have reseated the graphics card, restarted, system restored. Nothing seems to work. After unplugging and moving the computer to paint my room, the graphics card is not giving out a signal?? Im posting this with the monitor plugged into the onboard graphics - its fine but the resolution is smaller than im used to ect and i cant play my games using the onboard.
Would appreciate any help,