I assume you don't even get to the BIOS screen?
First thing is to get it to post.
Pull the battery on the CMOS wait 20 seconds, put it back in.
Try replacing the RAM chips, if there is more than one pull each one out independently.
reseat any PCI cards.
After that if it still won't boot I'd disconnect everything except the CPU and 1 RAM chip.
If it's still not starting, I'd try a different PSU, it's possible to have a PSU provide borderline voltage which makes it appear you have power, but prevents the system starting up or makes it unstable if it does boot.
If it still doesn't post it's probably a bad motherboard.
Basically what your trying to do is start with a minimum baseline and establish what works, then add the stuff back until it doesn't.