DIAGNOSIS This is almost always a tiny scrap of paper you can't see, or a sensor flag that didn't spring back into place. The sensors are in the paper path from the tray all the way to the back.
FIX Power off and unplug. Open the rear cover (the one on the back). Shine a flashlight in there and look for any shreds of paper. Gently pull out anything you see. Next, open the scanner unit (the whole top lid) and look at the paper path from above. Check the rollers near the duplex unit (if you have one installed). Manually rotate all the white rollers you can see with your fingers to check for resistance.
IF THAT DIDN'T WORK The jam detection sensor itself might be stuck. With the power off, locate the small, usually orange or black, plastic lever arms along the paper path (especially inside the rear cover area). Gently flick each one a few times with your finger to make sure it moves and snaps back freely.
HEADS UP Never use metal tools or excessive force to pull out paper. You can easily tear the paper and leave a fragment, or worse, scratch a roller or dislodge a sensor.