If you can't find it yourself, it's a job for a repair shop. Most likely candidates: PSU, motherboard, buttons and switches, cables.
Help! My pc was on the floor while I was cleaning my desk and sure enough a soccer ball struck the side of my computer and it was not the side my motherboard is on either and didn't get hit super hard either but shut off and won't turn back on now. When I hold the button down for a few seconds then release it then push it in right away it turns on for a split second then shuts down. Just pushing the button does nothing. Im so upset! It's an older built pc, msi 7507 board, dual core 2.66, 2gb of RAM, corsair gs700 pay and geforce 210 1gb card. Thanks for any help!