A backup of your FreeNAS settings and configuration is vital.
Hence, it is important to do this as you enable or change settings on any service, add any new disks etc.
This is done in System -> Backup/Restore
Select Download configuration and save the file on your PC hard drive (i.e. not just on your FreeNAS server).
Select Restore and choose the appropriate backup .xml file; they are all time stamped by yyyymmddhhmmss, allowing you to identify the latest backup.
FreeNAS will then reboot with the previously saved settings.