The Original Prusa XL is equipped with a Power Panic cable, which can recover the printer from a complete loss of power without needing batteries or an UPS. If the power panic cable is damaged or disconnected, the printer will not work, and show the text 'Power panic detected on startup. Check power panic cable'.
Error name: Power Panic
Error code: #17321
Make sure that you have a USB drive with the latest firmware connected to the printer. Go to the printer menu in Settings -> System -> Factory reset -> Hard Reset. The USB drive with the latest firmware has to be connected to the printer for the procedure.
After that, it will be necessary to go through the initial printer calibration again.