Remove Power completely, including from rear of PSU
Remove all USB headers and audio headers from MB, remove all SATA connections, and SATA device power inputs..
All you need connected are :
24 PIN MB power from PSU
4 pin power CPU PWR to mainboard from PSU
PWR SW to correct 2 pins on mainboard...
Drop down to one RAM stick in required slot (normally 2nd from CPU, check manual)
Reset CMOS or remove CMOS battery for 10 min, reinsert
Connect USB to a native header at rear of MB, not a disconencted front port or to USB3
Try again...
If no luck, you need a known good PSU to try...
Afterwards, the mainboard is left...