PSA error 1000-0123 or 2000-0123 is a RAM error...

1. Power off, unplug
2. Press/hold power button for ~15 sec
3. Open case and remove motherboard battery (check Service Manual for details)
4. Press/hold power button for ~30 sec
5. Carefully re-seat each RAM module (how many do you have?) in its slot.
   Make sure each one goes back in its original slot.
6. Reinstall battery (Might be time for fresh CR2032 3-volt coin cell battery, ~$2)
7. Close up and connect mouse, monitor and keyboard
8. Reboot