There will be a motor that causes the table to rotate via a series of belts, gears or pulleys.
Something has gone wrong with the assembly that transmits the motion.
On the subject of "is it worth fixing?" that's largely for you to answer. I can't see a service shop repairing it for under £40 - and my own unbranded microwave cost only £23 from Tesco 8 years ago.
Good luck...
Peccavi, May 2013
