|Have you checked the jumpers and switches are set correctly?|
Particularly the CMOS jumper and the OC Switch.
What memory type and speed and what slots are you using?
What graphics cards have you attempted to use?
Try the minimum hardware approach. Only connect the minimum hardware required to boot your system and try and get that working and then add other hardware later one at a time.
Remove unnecessary USB headers and cables not required. Put them back later.
Check all the cables again and ensure that they are all fully pushed home. SATA cables can sometime work loose.
If nothing works try and set up your jig outside the case with a bit of cardboard underneath the motherboard. If that works then there could be a short in the way the motherboard is connected in the case. Check there are no unused motherboard mounting studs underneath the motherboard.
☺ When everything else fails, read the instructions.