Ignacio Vila there is not much in terms of factory (hard) reset for a printer. In the case of the HP Officejet Pro all you have to do is to :
Turn the printer on, if it is not already on.
Wait until the printer is idle and silent before you continue.
With the printer turned on, disconnect the power cord from the rear of the printer.
Unplug the power cord from the wall outlet.
Wait at least 60 seconds.
You can restore the printer to the original network settings. To restore the printer to the original network settings on the printer control panel, press and hold the Wireless button and the Cancel button together, approximately five seconds, and then release the buttons.