Well, there's a lot of delicate components there that are sensitive to physical damage as well as electrical. Gotta be careful. Nuff said on that topic.
Now, assuming you didn't hose something like that, here's how you find out what's up:
Unplug the 'puter.
Unhook all drive cables from the mobo.
Unhook all case wires from the mobo except power button.
Remove all cards from expansion slots. Replace video card.
Remove all memory modules. Double-check to make sure your memory is compatible with your motherboard!! Replace the minimum number of modules possible. (dual-channel will be 2)
Remove and replace power supply connections to mobo.
Try to power on now. Don't forget to plug it back in.
If you see stuff on the screen, then what you have assembled so far is working. Power off, add one component, power on, and repeat the process until you find the offending connection/card/whatever. If it doesn't work, then you have a problem! Likely with your mobo, CPU, possibly vid card, or (long shot) memory. Inspect these components for damage, and try to replace what you can with spares to see if you can find the bad apple in your barrel. If not, probably need to RMA mobo and/or CPU.
Good luck!