Diagnosis
The motor unit has likely lost power. The most common cause is a blown thermal fuse in the motor housing.
How to Fix It
Unplug the unit. Let it cool for 30 minutes. Press the release button and separate the blending arm from the motor unit. Inspect the electrical contacts on both pieces for food debris or moisture. Dry thoroughly with a cloth. Reattach firmly and plug in. Try the power button.
If That Didn't Work
The thermal fuse may have blown permanently. This is a safety device to prevent motor burnout. The motor unit needs to be replaced. Contact KitchenAid support for a warranty or repair evaluation.
Heads Up
Never submerge the motor unit. The thermal fuse blows from overheating, often caused by running the blender continuously for more than a minute or in very thick mixtures.