If it is not getting to the beep stage I would suggest a power supply problem. I am assuming it beeped before, therefore there is a speaker connected.
Go back to the original build, use onboard graphics, unplug any USB devices except keyboard. Then very carefully unplug and replug all power connections. Pop out the CMOS battery, wait 30 seconds, replace.
Try starting the PC. If you still get nothing, try a different power supply. If it starts you will get a message about "Enter setup or press F1 to continue"
If you get nothing with a different power supply then your motherboard is faulty. But I suspect PSU because a motherboard with nothing connected will run POST (Power On Self Test) and beep like crazy and probably shut down again.
Good luck. I am away for the weekend now so I hope someone steps in if you need more help.