In order to reset a HP DeskJet F2420 printer, follow these steps:
Start the printer.
Keep pressing “Color Copy” and “Cancel” at the same time. While pressing these two buttons, disconnect the power cable.
Still, do not take your fingers from those two buttons.
Keep them pressed for around 30 seconds.
While still pressing them, connect the power cable.
And finally, keep pressing the two buttons until the printer won’t make any sound. It might take around five minutes.