Something installed wrong. Try reseating the memory. Got the 12v connector plugged in the motherboard? Try unplugging the drives from the motherboard. See if it'll boot then. Was it tested to be working before being installed in the case? Lots easier to troubleshoot out of the case rather than in it. I would remove board
from the case. Install just your cpu w/heatsink fan of course, memory, and video card. Unplug any extras like a usb header card, sound card, etc.... Have just the power switch
from the case plugged in. See if it boots so you can go into the bios. If it does then shut down and add the hard drive. Keep on doing it until everything your using is installed. If
you add something and it doesn't boot that's the problem.