ZFS maintains a cache of active pools and their configuration in the root file system. If this cache file is corrupted or somehow becomes out of sync with configuration information that is stored on disk, the pool can no longer be opened. ZFS tries to avoid this situation, though arbitrary corruption is always possible given the qualities of the underlying storage. This situation typically results in a pool disappearing from the system when it should otherwise be available. This situation can also manifest as a partial configuration that is missing an unknown number of top-level virtual devices. In either case, the configuration can be recovered by exporting the pool (if it is visible at all) and re-importing it.
For information about importing and exporting pools, see Migrating ZFS Storage Pools.