Switch the unit. let it rest for half an hour. While it is resting check the manual for any 'faults' list. Look for a 'reset' command - may be mean pressing several buttons while the machine is off and then with the buttons still pressed switch on the machine[you may need two hands and a nose or three hands for this ;-)] If still out of luck go to places for further info HP technical support team or www,fixyourownprinter.com
Usually answered in minutes!