DIAGNOSIS The low-voltage DC power to the formatter board and control panel has failed. The fan runs because it's on a different circuit from the main high-voltage power supply. The internal logic board isn't getting power.
FIX Unplug the printer from the wall outlet. Wait 60 seconds. Plug it directly into a known-good wall outlet, not a power strip or UPS. Power it on. If the panel stays dead, the DC power supply on the formatter board is faulty. The formatter board (part number RM2-4456) must be replaced.
IF THAT DIDN'T WORK Before replacing the board, check the internal connections. Unplug the printer, open the rear door, and remove the duplexer. You'll see a panel with screws on the right side (printer's right). Removing that panel gives access to the formatter board. Check that all cables, especially the wide, flat one to the control panel, are firmly seated.
HEADS UP This is not a user-serviceable fix if the board is bad. There are exposed high-voltage points inside even when unplugged. Do not attempt to repair the power supply circuitry. Contact HP support or a technician.