Pull CMOS battery, Reseat.
Unplug and replug all connections if you haven't already.
Test with another power supply if you can.
On various occasions when I have installed/cleaned my case i forgot to connect the 4pin CPU power line and it would power on and spins my fans and the lights would come on but nothing else would happen. I don't, however, remember if it gave me beep codes.
If that doesn't work pray there are wiser gerbils than me to give you ideas.
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