I had a problem with a PC not booting up. Brought it in to work for the IT guy to take a look at. Having eliminated the PSU as the problem, I was startled to see him dismount the CPU from the mobo and whack in another one. When the PC still failed to boot, he concluded it was a faulty motherboard. He was right. But I guess to do that, you have to have a spare CPU of a compatible type lying around...
Some things to Try first
1. unplug all USB equipment and retry
2. unplug DVD drives and retry
3. unplug hard drives see if it boots to an error message
4. try one memory stick at a time
5. substitute the PSU (power supply unit)[may be good enough to run small powered fans but not load of drives and motherboard.]