the HP LaserJet 2100, 2100n, and 2100tn Printers with firmware v 19991018 or higher can cold reset the HP Jetdirect using this method: Hold down the small JOB CANCEL button. Turn the printer on; continue to hold the JOB CANCEL button for seven to ten seconds, then release. Do not hold the JOB CANCEL button longer than 10 seconds. Printers with older firmware can cold reset the HP Jetdirect; however, the page count and serial number on the printer’s Configuration page will be lost (the serial number is, however, also printed on the back of the HP LaserJet 2100 printer).

This procedure is for reference only and should be avoided if possible. Print a Configuration page. This will ensure that a hardcopy of the current page count and the electronic serial number is available. Turn off the HP LaserJet 2100 printer. After the power is off, press and hold JOB CANCEL.
Turn the power on, and continue to hold JOB CANCEL until all of the LEDs come on or for five to 20 or more seconds after all of the LEDs come on.