Were you able to remove the bobbin and the bobbin case from the machine? If you can't get them out, you'll need to take it to the dealer.
If you can get the bobbin and the bobbin case out, see if the thread is stuck in there. If so, just gently pull it out.
If you can get the bobbin and the bobbin case out and a thread is caught in the part that goes around (I think it is called the race) and you can still get a grab of the thread, gently but firmly pull on the thread (I use a pair of tweezers) while moving the hand wheel around slowly to see if that will loosen it up. Sometimes I move it back and forth a little. If you still can't get it out, I'd take it to the dealer.
If you do get the thread out, be sure to clean the area in case there's bits of thread laying around. If you machine is still not working correctly, you'll probably need to take it in too in case the timing was thrown off somehow.