DIAGNOSIS: You have a severe jar-to-base misalignment or jam. The motor's thermal overload protector has likely tripped to prevent burnout because the blade assembly is mechanically locked.
FIX: Unplug it immediately. Let it sit for 30-60 minutes to cool and reset. Remove the jar, then manually rotate the blade assembly inside the jar by hand. If it turns freely, reseat the jar on the base with a firm, deliberate twist until you feel it click into the drive socket. Plug it back in and try a low speed.
IF THAT DIDN'T WORK: If the blades inside the jar *won't* turn by hand, you have an internal bearing seizure. For the K150, the entire blade assembly/coupling unit must be replaced—the jar is not user-serviceable. Call KitchenAid for a replacement jar.
HEADS UP: Never run it for more than a few seconds when it's humming. You'll cook the motor windings. If it repeatedly hums with a correctly seated, free-turning jar, the motor capacitor is failing and requires professional repair.