The blade adapter or drive key has sheared. This is a safety feature to prevent motor damage.
Disconnect the battery. Tilt the mower.
- Attempt to turn the blade by hand. If it spins freely independent of the motor shaft, the shear pin or bolt has failed.
- Remove the blade bolt (part number 9) and the prevailing torque hexagon lock nut (part number 28).
- Remove the blade. Inspect the blade hub and the motor shaft for a sheared drive key or damaged splines.
- Replace the blade (use part number 1, 2, 26, or 27 as appropriate) and all fasteners. Torque the blade bolt to specification.
If that didn't work, the DC motor (part number 201-8) shaft itself may be broken. This is less common but requires motor replacement.
Heads up: Hitting a solid object is the usual cause. Always replace with genuine EGO replacement blades and hardware.