check all the obviouse stuff, cables power etc.. the usual, thou i expect you've already done this.
next check up on how well the internal stuff is connected, check the gcard is in, as well as the power to the mobo?
try checking the moniter on another comp? if its an old one it may have bust.
you could well have messed somthing up when building, this could be a range of thinks ranging from drastic to slap-on-th-fore-head silly. go through ever thing you need to make it run, there may be an online guide for baisc guidelines you can check against.
hope its nothing to seriouse.