DIAGNOSIS The lid safety interlock isn't being detected. The magnetic sensor in the lid handle isn't aligning with the base.
FIX Reseat the lid. Twist it clockwise until you feel a distinct click and see the handle icon line up with the power icon on the base. The 'lid' message should disappear immediately.
IF THAT DIDN'T WORK Check the small silver magnet inside the plastic protrusion on the underside of the lid handle. If it's missing or cracked, the base can't sense the lid. You'll need a replacement lid.
HEADS UP Don't try to trick the sensor with another magnet. You'll bypass the safety mechanism for the extremely sharp blades.