Their will be a smal motor under the turntable.
First ensure plastic part that moves the glass tray is not worn or damaged as this is the usual as the motor drives this directly.
If this is alright the motor may have been damaged by liquid causing corrosion and access varies depending on models but usually you will find an access plate and may have to cut out the notches to remove and when refitting use a screw.
sparky
November 2005