Do you have any other display device you could plug it into (your TV, another monitor, etc.). Many devices have VGA hook-ups. If you have another computer, you could try that computer in the monitor. I would also try a different VGA or DVI cable (not sure what type of connection you are using). I would rule these things out before focusing on the GPU. Your monitor could be going bad.
If you rule these things out, you may be looking at a bad video card...which requires a replacement video card.