Hello Lindsey,
Release the stop motion and see if the balance wheel moves, tighten it back up, force the balance wheel backwards because there may be a small piece of thread stuck in the raceway that you can't see, if that does not do the trick then take off the top cover and inspect the patter camstack and see if there is a split that goes down it, this is quite common, if there is then the machine will be useless unless you are prepared to spend a fortune on replacing the camstack, e-mail me with your findings.
drivewithken@blueyonder.co.uk, May 2008