The next thing to do is unplug from mains take the cover off, earth yourself ( so touch a radiator ) take a peek inside case.
Without touching any metal contacts, or metal on the PC check the video card is seated ( give it a gentle push into the slot) same with the memory sticks.Try to judt touch the card edges not the metallic bits at all
Press v gently on the CPU to make sure it's seated ok.
Check no loose dangling wires !
Leave lid off ( don't put your hands inside case when plugged in ) power on - does power fan come on ? If not check plugs n fuses. If it does does CPU fan come on ?
Do you hear HDD's spin up
Do the floppy / HDD / CD lights on the case light up ?
Give us a run down of what happens ( you should at least get beep ( hopefully )
If you don't get anything at all definately no power to motherboard so it is check wiring plugs etc.
Above all if it's plugged in at the mains don't stick your hands in - make sure unplugged before you look.
Well yes, I am sure you have checked and checked again, but ... did you by any chance disconnect any of the drive ide cables, including floppy, if so might you have put cable back on the wrong way round? Did you actually remove mem sticks and then replace them, rather than just giving them a bit of a push down?
Clutching at straws a bit, but I can only suggest - check again.