Westwood T1200 no drive?

Westwood T1200- help please. No drive and had to push back to garage. Drive belt seems to have come off as if I engage gears can’t push. Was cutting, stopped to put in reverse and then lost all drive. Cutting deck still works and drive belt loose??
I’m no expert or mechanic so all advice welcome as a hobby mower… live in Bucks, MK 19 if anyone local??
Thank you
Keith