It sounds like you have something threaded wrong. Now you have threads all caught up in the bobbin case. You need to do a bit of surgery to untangle it all. Remove the needle plate and clear all the threads. After you get it all put back together, never turn the hand wheel backwards, that helps to cause thread jams.
The bobbin goes in with the thread going around to the left. Needle goes in with the flat to the right.
Here's a diagram to thread the top of the machine, scroll down the page until you find your model.
I hope this helps.
Singer 99K,66-1,201,15-90,237 and several featherweights and 301's