1. Was it working before? if so, boot in to safe mode and see if you get an image.
2. If not, did you just put it together yourself? Did you use a static-neutralizing wrist strap?
3. Can you swap out the monitor for a known good? If it works, the monitor may be bad or have some wrong settings.
4. Can you swap out the vid card for a known good? ? If it works, you my have a bad video card, or have some wrong drivers installed, or your drivers are corrupt.
**NOTE: If you are handling any components use a damn static wrist strap to neutralize the static electricity, dont work on the carpet and always place components in a static-free bag**
If you can't adhere to the above "NOTE:" or figure out what i am typing about , take it to a computer shop and get your bread broke for them to figure out some pretty basic you could have done yourself.