Is it plugged in (at both the wall and the PSU)? Is the PSU switched on (assuming it has a switch)? Are all the appropriate power cables plugged in (24-pin and 4/8 -pin to motherboard, possibly some to graphics card)? Is the case switch connected to the correct pins on the motherboard for the power (not reset) switch?
------------------------------i5 2500K, GA-P67A-UD3R, 12 GB RAM, 1 TB Spinpoint F3 & 250 GB WD HDDs, DVD writer, Dell U2311H 23", Philips 170B6 17" & Philips 150B4 15" monitors, Dvorak keyboard, openSUSE 11.4 with Windows Vista Ultimate in a VB VM.