I have three self-built computers which will power up right now but not post. Two of these computers were working previously, but were bumped and then stopped working. The other is a new computer. I have stripped them down to just a power supply (which has been tested to verify that it works), a motherboard, a CPU, a heatsink, a fan, and the speaker. The system powers up - fans turn on, lights on motherboard turn on, hard drives click if connected- but no post occurs. Any help would certainly be appreciated.