Hi Ann,
If you take the plastic covers off the back of the machine, you can clean round lots of the workings and replace the grease/oil. By working the mechanism back and forth, you may be able to free it. If it continues to give problems, it is probably a case for an engineer. Under the needledbed there are a series of very thin plates which move to select the needles. Over time they stick together, effectively jamming up the works. It needs the expert touch of an engineer to free them as they are so thin and delicate.
Hope this helps,
Sue, August 2013
