There is a direct mechanical connection from the hand wheel to the needle bar.
Since you have the face plate and top off, turn the hand wheel and watch the main shaft. Does it rotate? If not then the stop motion clutch or something isn't working right.
If yes look over on the needle bar side. There is solid metal parts connecting the main shaft to the needle bar. Follow the motion and look at each piece until you find the one that isn't moving. There is no plastic in there to break.
Joe