DIAGNOSIS The scanner carriage is stuck or the flatbed cable is damaged. A loud humming noise confirms a mechanical lock.
FIX Unplug the printer. Open the scanner unit. Look at the right side. You will see the scanner carriage and a white plastic screw drive. Gently try to move the carriage by hand toward the center of the glass. It should move smoothly. If it's stuck, clear any obstruction.
IF THAT DIDN'T WORK The flatbed cable that connects the scanner to the main board may have come loose or failed. This requires disassembly of the scanner unit to inspect. The connector is on the left hinge side.
HEADS UP Do not force the scanner carriage. If it moves but the scanner still fails, the stepper motor or control board is faulty. This is a complex repair.