Hello Adrian
Are you able to enter the system BIOS or BMC/ESM via F2 or CTL E? I would recommend clearing NVRAM via jumper on the system board. Power down, remove power cables, move NVRAM jumper to CLR, and then power on the server for a few minutes. Use the same steps to move the jumper back over. If the issue persists then you will want to start taking the system to minimum to POST. These are the items required to POST:
System board
Processor (with heatsink) in socket 1
One stick of memory in slot 1
1 power supply
Control panel (to turn on system)
Sideplane
Both PCI risers in matching configuration
Thanks