Diagnosis
A tiny piece of paper tore off and is hiding in the paper path, or a sensor flag is stuck in the 'jammed' position.
How to Fix It
Power off and unplug the printer. Open the rear cover. Look deep inside near the fuser exit area for any shreds of paper. Also, check the duplex unit (if you have one installed)—flip down the green lever on its side and pull the unit out to inspect. Remove any debris. Plug back in and power on.
If That Didn't Work
The jam detection sensor might be faulty. Try a hard reset: unplug the printer, wait 2 minutes, then plug it back in while holding down the 'Go' button. Keep holding until all LEDs light up, then release.
Heads Up
Never yank paper out backwards during a jam. Always follow the direction of the arrows printed inside the covers. Forcing it can break sensors or leave fragments that cause phantom jams.