A device within a ZFS pool failed and could not be reopened. Running 'zpool status -x' will provide more detailed information about the particular failure:

# zpool status -x
  pool: test
 state: DEGRADED
status: One or more devices could not be used because the label is missing or
        invalid.  Sufficient replicas exist for the pool to continue
        functioning in a degraded state.
action: Replace the device using 'zpool replace'.
   see: http://www.sun.com/msg/ZFS-8000-4J
 scrub: none requested
config:

        NAME                  STATE     READ WRITE CKSUM
        test                  DEGRADED     0     0     0
          mirror              DEGRADED     0     0     0
            c0t0d0            ONLINE       0     0     0
            c0t0d1            FAULTED      0     0     0  corrupted data

errors: No known data errors.

The 'see:' field will direct you to a more detailed article explaining why the pool could not be opened.