There's a gearbox assembly attached to the agitator drive spindle. It sounds as if it has failed. You can check it by rotating the agitator in both directions. If it turns if both directions, it has failed.
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Yes, unfortunately, it does mean it's failed. Replacing the gearbox is a major undertaking, and the part is expensive. I suggest that you contact a local service person for a phone quote for the job, before proceeding further. It will help you decide what to do next.
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restaurant equipment technician
20+ years experience. I'm now employed at a central kitchen, which serves 5 restaurants.