Snapshots which were created before the restore of an Auto Backup cannot be used for Roll Back via the Customer Control Panel. 

If you want to get access to that snapshot, please contact customer support.  

You can raise a ticket with the customer support team via the Customer Control Panel here.

Snapshots created after the restore of an Auto Backup are fully accessible.

You can follow our guide here on how to rollback to a Snapshot of your VPS.

However, please note that rollback from snapshots is not available while a backup is restoring and while backup creation is in progress.