DIAGNOSIS The scanner unit (the flatbed) isn't initializing. This is often a mechanical lock-up or a communication glitch between the scanner and the main printer logic board.
FIX 1) Turn the printer OFF using the power button. 2) Unplug the power cord from the back of the printer for 2 full minutes. This is a full system reset. 3) While it's unplugged, manually move the scanner carriage. Gently push the white scan bar (under the glass, on the right side) from side to side. It should move freely. 4) Plug the printer back in and turn it on. Listen for the scanner carriage to move to its home position. This often clears the error.
IF THAT DIDN'T WORK The flatbed cable might be loose. This requires more disassembly than most folks should do. If the power reset didn't fix it, and you're sure nothing is obstructing the scanner's movement (like a paperclip), it's likely a hardware fault.
HEADS UP Don't keep power cycling the printer rapidly. If the scanner is physically stuck, you could burn out the drive motor. The unplug-and-wait step is crucial to let all capacitors drain, which performs a true reset.