Any chance you might have a short somewhere? I trust you used the standoffs between case and motherboard?
I'd check and re-check all connections to be sure something hasn't worked loose - power cables, HD cables, re-seat RAM modules, re-seat all cards incl. video, etc.
If that fails try going to barebones - unhook ALL drives and USB devices and go to one stick of RAM only, then try a different stick of RAM, etc. - try to narrow down culprit.
If that fails then carefully rebuild the system outside of the case and attempt to boot barebones. If THAT fails then there is a good chance one of your components has died an early death, they tend to die early on if they were defective. When you get a system as unresponsive as that then the likely culprit would be motherboard or PSU, of course of the two trying a different PSU is the easiest thing to test and should be ruled out first. If that's not it you may have to RMA the motherboard and hope that was it.
And did you remember to hook up the aux CPU power connectors?