I had exactly the same problem with my PC last year - no POST but all the fans etc spun up just fine.

Took it to a hardware guru workmate who suggested it was the power supply.

I wouldn't believe him, since everything (fans, HDD etc) were coming on, but he swapped in a spare PSU (of equivalent wattage) and it booted just fine.

So I'd suggest trying the PSU if you can get hold of a spare.