if it is the timing belt then this should be fixed by a recognised machanic because if the timing is out it can cause damage to the engine.
if the fanbelt has come off you usually need to slacken off the alternator to replace it.
it is unusual that the belt comes off by itself and there maybe some other issue such as a loose part. i would speak to a dealer to ensure you are not wasting time and money.
Answers & Comments
It is dependent on which belt has come off,.
if it is the timing belt then this should be fixed by a recognised machanic because if the timing is out it can cause damage to the engine.
if the fanbelt has come off you usually need to slacken off the alternator to replace it.
it is unusual that the belt comes off by itself and there maybe some other issue such as a loose part. i would speak to a dealer to ensure you are not wasting time and money.
I hope this was useful