You mean one of these ... .http://www.sunsetmtb.co.uk/shop/inde...product_id=983

Er, take it apart and look?.. I suspect the rear freewheel has gone out. might need it rebuilt, or replaced

I'd enlist the bike shop's help, since I cannot see anything you are talking about,
and I have no Idea about what tools you have available ..
if its a screw on Freewheel you need the freewhweel removal tool specific to the brand of Freewheel they installed.