Couple things to try: Reset the BIOS.
If that doesn't work, unplug the main power connection to the motherboard, press the power button the PC case, unplug the power supply from the wall socket, reconnect the motherboard power cables, plug the PS back into the wall and try powering on the system again.
I've had troubles like you describe before, the fans would come on with the HDD activity light- but nothing would actually power up. Doing the above, the system would work again.
I'm pretty sure it was due to a faulty surge protector, replaced it and haven't had the problem since.
-SW