Recheck all your connections.
You don't need a DVD drive to boot so unhook it's two rear connectors.
Are you getting information on the screen?
If so your video card and CPU are likely just fine.
As said, you need to describe what's going on with the screen and exactly when things fail.
Basic troubleshooting is to unhook everything not needed, if the problem still exists you know where to look. If it goes away, add things one at a time.
For example, if your problem still exists after removing the DVD drive then possibilities include:
1) CPU or CPU heatsink not mounted correctly
2) CPU fan not spinning
3) RAM (try one at a time in the correct spot; likely not the issue)
You get the drift.. It's called basic troubleshooting but it can be confusing.