If you reset your configuration to default or you re-install and you want to keep the old pool you MUST import the old pool from command line before re-creating the pool from FreeNAS WebGUI. By creating the pool from WebGUI you destroy the old pool and create a new one.
If you don’t know the name of your old pool run the following command to find all pool’s available:
zpool import -f
Once you know the name of the pool import the pool:
zpool import -f [pool name]
Make sure that the pool is mounted:
mount | grep zfs