what motherboard are you using? there is a big possibility that your front panel i/o connectors are placed wrongly for the power switches and what not. can you please recheck them.
other factors to consider :
1. you mustve changed something weird on your BIOS by accident. so try to give it a CMOS reset on the 2 pin jumper. dont forget to return the jumper to its original 1-2 position. not on the 2-3 pin.
2. the Powersupply is faulty
3. the motherboard is faulty
4. bent pins in your CPU socket mount