It may be that the needle bar, the bar that makes the needle go up and down, may have been jammed. It makes it too short so that the thread cannot loop around the shuttle. The solution is to loosen the screw holding the bar and side it down an infintismal amount to see if it helps. If that produces a knocking noise, the bar is too long. It is best to have this done by a repairman, but I have done it myself a number of times. Mind you, this is only one possibility. Good luck.