Here's a list of things to check and try:-
1. Check you've mounted the motherboard correctly, i.e. no remaining pillar supports for the old board when there aren't any matching holes in the new one.
2. Disconnect all peripherals from the board, i.e. drives etc., and disconnect their relevant 5v/12v power connectors.
3. Pull out all add-in cards from the motherboard slots baring the video card.
4. Carefully re-check the motherboard connections to the front panel, RESET, POWER-ON, etc., etc., and if in doubt disconnect and leave only the POWER-ON.
5. I assume you put a nice thin smear of heatsink compound on the CPU before you clipped the heatsink/fan in place?
6. Clear CMOS once more, and try powering it up.
7. If you can get back to the point where the fans start whirring away, but still no signal to the monitor, try removing the memory (with PC switched off), and see if powering up again produces a series of bleeps from the speaker.
8. If in 7 above, there are no bleeps from the speaker, you could have a dead CPU.
9. PSU rating? 300W? 400W? If lower than that, then you might have blown it if it fails to come on at all now.
Ok, keep us posted on your progress...
ROGER - G0AOZ.