That's the main message when I tried to power on the Ubuntu 19.10 which is installed with the ZFS filesystem. A few hours ago in my CCNA class, it worked fine. Now when I returned home, I see this message:

Failed to import pool 'rpool'.

Command: /sbin/zpool import -c /etc/zfs/zpool.cache -N 'rpool' 
Message: cannot import 'rpool': one or more devices is currently unavailable. Error: 1

Manually import the pool and exit.
Screenshot of the error message: