Good afternoon. I find these old gals and most of the time it has so much dust and thread in and under the bobbin hook that it can no longer move. You must take it out (on the 99 you must release it from the bottom of the machine first) pull it out (take photo before you start taking it apart so you remember how to put it back together) clean all the "stuff" out, put a drop of Singer oil on the shaft and rework it. Once you get the hook assembly out, you might try moving the wheel again and it shoudl be ok. Otherwide, don't force anything. A few drops of oil and move back and forth 1/8" inch at a time each way until it frees up. I've also found that once these ole gals are free they sew like a gem. Check my model 15 on www.livingwellforless.net it was frozen up and now sews strong and smooth....
Capt Jack :)