Try cleaning, and then use a different bobbin and see if you are still having the problem. You may have a piece of thread stuck somewhere.

Hi I once had the same problem and after trying all that has been suggested I took to the dealer and it was the timing . Thought this might help . 4U

Also, make sure your bobbin is in correctly, and in the tensioners properly. If you didn't get it in all the way, the thread could have jumped out of the tensioners, causing lots of problems, ask me how I know, hee hee hee. Good luck, Elizabeth!! Hugs, Marji