The hard reset erases passwords and any other credentials such as use for emailing or faxing. Some printers have their page count reset. The only way to revert firmware is to flash it from an executable or an image of the firmware using a hardware tool to do the flashing. If your printer is under warranty possibly an HP agent can restore the firmware if it has been corrupted. This assumes there is enough of the existing firmware working that the printer can update itself.
This is a user to user forum. What problem are you having?