Hi,
Once reloading the paper into the tray, open and close the cartridge access door to resume printing and the other side will be printed.
As a reference see page 46 of its user guide:
Inspect the printer, determine the error, and then correct the error.
If the error is the absence of paper in the proper tray or the printer is in a manual feed mode,
put the proper paper in the tray and follow the onscreen instructions,
or open and close the print-cartridge door to resume printing
http://h10032.www1.hp.com/ctg/Manual/c05185433
