Is your monitor on? if it is a notebook try this:
press and hold the FNC (function) key on your keyboard
scan the F keys at the top for a picture of a moniter (or something. usually F4 or F5)
press that key until the monitor turns on.
If that doesn't work, try booting in safe mode. You may (or a virus may) have accidentally (or in the case of a virus, purposely) disabled your video card drivers. You will somehow need to reinstall these if this is the case.