Did your car Battery was drained out completely ?
To help you with - Authentication key is a 4 digit number.
Suggest to check Owner Manual as given. Generally the authentication codes, key numbers are mentioned in it in the service booklet
In case - you still could not get it - suggest to contact or visit Renault Workshop. Probably - they can assist you as they need to punch in unique music system serial number in the Software - basis which the authentication key can be found.
No charges should ideally be charged by workshop for same.
Hope should help !!