No beeps, hmm something wrong with hardware there.
1. remove/reinsert memory
2. remove/reinsert processor
3. remove all ide cables and see if boots from
4. remove/reinsert video card
5. remove all cards except video and if it boots
replace cards til culprit found
It could be the real obvious, the on/off button. I presume its an ATX PSU, so try shorting the two pins with a screwdriver bypassing the switch.