Could it be the feed dogs? Are they down, perhaps? Watch to see if they move when you sew " on air" for testing.

All kinds of things:

1 - bobbin tension too tight
2 - bobbin inserted backwards
3 - Needle inserted backwards
4 - Upper tension too loose
5 - bobbin thread not completely in the slot
6 - bobbin holder not completely inserted and locked in

check to see if the top thread has come off of the thread take up lever, that will cause the machine to jam up also..
like someone else said, a bobbin inserted backwards will jam up the machine also..

Please bobbin winding properly and change the needle then check the timing.

I hope you solved your problem but thought I would add that I agree with serger... that kind of problem usually means the thread isn't threaded right on the upper part of the machine and it usually is the little eye that moves up and down....