There could be several things wrong. Here's a list of potential reasons why there is no display:
1. RAM failure
2. GPU/Chipset failure
3. PSU failure
4. CPU failure
5. Short circuit
6. 4/8-pin CPU connectors not plugged in
7. Monitor signal cable failure
As you can see, the list is quite long. Start with the link in my signature. If the checklist doesn't help you, post back with the checklist as your template. State what steps in the list you've completed and the result of said step. The more info you provide, the easier it will be to assist you.
If you can try the monitor on another system. If it works the likely culprit is the GPU. Another thing to try is use the VGA connection between GPU and Monitor to see if the DVI port is functional on the monitor. (Or the other way round if you are using the VGA port.) The other main suspect would be the PSU. It may be that if the screen is attached the GPu tries to fire up and the PSU may not be delivering good power. Make sure your PSU is plugged into the GPU properly. It may be a fail safe cutting in to protect the GPU from under voltage.