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 a UPS. If the power panic cable is damaged or disconnected, the printer will not work.
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 (USB with FW needed).
After that, you will need to go through the initial printer calibration again.
In case that does not solve the issue, contact our Customer support.