It's what I'd do...
Grab a FreeBSD rescue CD (11.1 disc1 at the time of writing), boot into the bootmenu, escape it (ok> prompt) and then enter
lsdev
. This will list all available boot devices and the beauty of it is that it will also recognize ZFS pools. If that still mentions something then there's a chance the OS is still accessible. I wouldn't try to boot with it at this point, for safety reasons.
Go back into the boot menu and boot the CD or simply issue
boot
and fire it up. Start the live CD. Log on as root.
Now, there is a chance that the ZFS version on FreeNAS differs than the one on FreeBSD. This is one of the reasons why
SirDice points your attention to that link: because of the changes in the OS it is definitely possible to cause more damage. For example: what would happen if the ZFS versions differ and by accessing it with FreeBSD you somehow change that, which could then cause issues for the FreeNAS environment?
Once booted first issue the
zfs import
command. This will load the required drivers and list the pools available to import. You'll either don't see anything (in which case you got a problem) or... who knows. I hope for
zroot.
Then try:
# zpool import -o readonly=on -fNR /mnt zroot
This will try to mount your pool readonly (no risk of causing further damage) and without mounting filesystems (to be extra careful). Be sure to replace
zroot with something else if applicable. If this command actually works then try
zfs list
, this should list all your filesystems.
You can then proceed by making backups. Either by using
# zfs send
or mounting the filesystem and then using other means to restore your data,
tar and
ssh for example.
For reasons mentioned above I cannot provide further help beyond this point because of the differences in OS's. But I hope this can be useful.
Keep well in mind: when writing on these forums I
always keep FreeBSD in mind. This works on FreeBSD so if it doesn't work for you then, well, you already know the reason