Remove all external devices and power from the machine.   Press and hold power button for 15 sec. to remove residuary power. Open side panel and perform CMOS reset via jumper, give it a fresh CR2032 batt while working on motherboard. Reassemble everything and test system again. If machine regain its functioning, access BIOS firmware and restore settings.
We’ve just had a powercut. The UPS kicked in and I hibernated immediately. But now when the power’s back, it won’t start up. I have the power button flashing amber with 2 then 1 flashes, which appears to be a “bad motherboard”. I realise the PC might well be toast, but is there anything I can try before replacing it?
OptiPlex 3050 power button flashing “bad motherboard”