ok, so the pool is there, which means the problem is with your zfs.cache, it is not persistent and that is why it looses its config when your reboot. what I'd suggest to do is to run:
zpool import zfsPool zpool list
And check if the if it is available. Reboot the server and see if it comes back, if it doesn't then perform the same steps and run:
zpool scrub
Just to make sure everything is alright with your pool etc.
Pls also post the content of:
/etc/default/zfs.conf /etc/init/zpool-import.conf
Alternatively, if you are looking for workaround to this issue you can set it of course as follow.
Change the value in from 1 to 0:
/etc/init/zpool-import.conf
and add the following to your /etc/rc.local:
zfs mount -a
That will do the trick.