Try a new signal cable (VGA or DVI cable, whichever is appropriate). If that makes no difference, the graphics card may need re-seating in the slot, or it may be faulty and in need of replacement.
It would be helpful to post your complete system specs or make/model of your computer. If it says "no signal" on the monitor screen, the cable & monitor are probably fine. The problem is the monitor isn't receiving a video signal from the video card or onboard video. That could mean the card is bad, but it could also mean the problem lies elsewhere. What happens when you boot the system, are there any beeps? Do you hear the cooling fans spin up? Do the LEDs light up on the case?
