The chances are that the drive belt has either broken or come off the pulleys on the underside of the turntable platter and the motor.
You will need to access the underneath of the turntable to do this.
How you do so varies from one model to another - some you can just lift the top plate off the wooden base, others you will have to turn the whole thing upside down and unscrew the bottom.
With some turntables you lift off the rubber mat from the platter to expose a couple of little holes through which you can access the drive system.
If the belt has not actually broken you should be able to put it back onto the pulleys. However, the fact that it has come off indicates that the belt has stretched and you should replace it. There are plenty of suppliers on the internet (Amazon is usually worth a try) where replacements can be found