Deluder,
Post code 50h in AMI BIOS is initializing USB.
Try removing any USB devices attached or internal card readers.
If that does not work remove everything from the motherboard (all cards, disks etc.) then try again. If you get beepcodes start replacing untill you find the offending device.
If this happened during a BIOS update you may be able to restore the system by placing a floppy with a good bios for this machine in the floppy drive, remove the battery and power, wait at least 1 minute then try to boot again. Some systems will attempt to load the BIOS it finds in the floppy drive at this point.
If all of this fails it’s probably time to replace the board. I believe a standard ATX board will fit but I would test the alignment of the external connectors and mounting screws with another motherboard you hopefully have lying around.
Good luck.
-Fred