Underneath the turntable will be a drive mechanism - motor and drive belt probably but what do I know?
To make progress - the mechanism will need to be inspected and to inspect it the covers need to come off - but be warned.
Users are not encouraged to get inside microwave ovens - very high voltages and radiation lurk inside these things.
You have four options...
Attempt a repair yourself if you have the confidence
Have somebody else repair it for you
Buy a replacement
Use it without the turntable