The main shaft is threaded into the motor housing where the prop is and it may have broken free. Loctite will fix this problem easily. If it's not the bottom of the shaft, the top has a gear that is threaded onto the same shaft and may have come loose. Pair of pliers and loctite will also stop this. But this is from my experience with Motorguide trolling motors and the top gear was metal on both. Hope this helps and good luck.

Rollcaster..............................