Hello Emma,
Your machine may need retiming but if nothing is moving under the needle plate the nylon gear may have crumbled away wich is very common on Singer machines of that age, take the top off the machine first and look at the two nylon gears in front of you just about in the middle of the machine, turn the balance wheel and see if the bottom gear moves, if that is okay take the bottom off the machine and look at the two nylon gears under the bobbin case area and see if they are both moving, if any gears are broken they can be replaced quite cheaply by yourself and the machine retimed by yourself, I can attach timing instructions if you e-mail me.
Ken
drivewithken@blueyonder.co.uk, August 2008