Diagnosis
The scanner carriage (the part that moves under the glass) is hitting an obstruction or its movement is jammed. Often it's a small piece of paper, a paperclip, or a dried blob of glue that fell into the scanner bed area.
How to Fix It
Unplug the printer. Open the scanner lid and the document feeder (if it has one). Look along the white scanner strip (the guide rail) on the left side inside for any debris. Gently wipe the rail with a dry, lint-free cloth. Manually try to move the scanner carriage from the center; it should glide smoothly. Remove any visible obstruction carefully.
If That Didn't Work
The carriage might have come off its track or a plastic part inside has broken. You'll likely need to disassemble the scanner assembly to re-seat it, which is a delicate process best left to a repair shop if you're not comfortable.
Heads Up
Never force the carriage. If it's stuck, applying too much pressure can strip the plastic gears or break the drive belt, turning a simple fix into a major replacement.