Hello Stephanie,
Check the obvious first and make sure the reverse is not stuck in, if you can take the top off the machine you will see just right of the centre a two pronged fork with a cam sitting in the middle of it, in the centre of the cam there is a screw, you need to slacken off that screw and move the cam slightly one way or the other and then tighten the screw and test sew and keep adjusting until you get the stitch right, if you can't get into the top or you do not have the cam and fork there, you will need to take the bottom off the machine and the cam should be near the bottom of the teeth, if you have a two piece machine and you can't get to the inside it will be a shop job I am afraid.
drivewithken@blueyonder.co.uk, January 2008