Take out the bobbin case first of all to make sure you do not have a piece of thread stuck in the raceway, quite common, with the bobbin case out see if the machine works when you press on the control, if you can take the raceaway that is even better, if the machine is still tight after that then somewhere on the machine it is seizing and needs oil probably in a place that you can't get to.
Ken
drivewithken, December 2009