Hello,
Reset the CMOS as per the instructions in the manual (1.1.7).
If still no signal, remove the CPU and check for any bent pins, reseat checking for even mounting pressure.
Check that stand off screws surrounding the CPU and DIMMs aren’t over torqued and report back*
*
13900KS / 8000 CAS36 / ROG APEX Z790 / ROG TUF RTX 4090