When I've had this happen, I've had a 'nest' of thread under the material which is jammed into the needle plate. What I've ended up doing is removing the needle and the presser foot and cut the needle thread (to have more room to work) flip up and remove the needle plate and cut the bobbin thread. Now you can take the 'mess' in your lap and start trimming away that lump of thread under the needle plate. Depending on how much extra thread you have you may already have a hole in your material. Just do your best to fix it. Now what caused the problem. Your needle thread was not threaded correctly. (Sound like it should be the opposite, right) Rule of thumb is if you have a problem underneath, the problem is above and if you have a problem with the top thread the problem is the bobbin. Good Luck and happy unsewing.