How are the impellers mounted? If it uses tapered collets, you could have broken one impeller loose and it is not spinning. If the impellers are key driven (suggested by your question) with split rings or snap rings to locate, I would not expect there to be room to drop far enough to come completely clear of the key. Is there any change in the vibration to indicate a rub or cavitation. I would expect a failed wear ring or restricted suction before a dropped impeller. But either is possible. If it is in an open sump, I would pull it up and check for 2x4's and bucket lids. If it is in a can, I would check for suction strainers or dropped valve gates.
Johnny Pellin