|Try this first maybe. Be sure to remove all un-needed devices for the last ditch way to test. I strip out all cards and drives except one and test. If works then add in cards. I sometimes even remove memory to one stick.|
A common way for a system to power up is the power button connects through the motherboard and back to the ps. The PS ramps up and at some point a latch needs to happen. This gets tricky on some combo's of ps and mb.s Since you say it did work we can only suspect two items. One is PS and the other is MB. As I said earlier there are know issues with in some cases both being good but fails to latch because of timings.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10