It could happen that one of the storage processor of your SAN displays as unmanaged in GUI.
This can happen after a SAN reboot and is usually related to the UPS powering this SP.
Since the communication between the UPS and the SAN is broken (not available), the SP goes on unmanaged and disable the cache.
In order to solve this, you'll need to stop the SAN (and everything running on before), stop the UPs, restart the UPS and let it do the check opreations (< 1 minute), and finally start the SAN.
Everything should be ok.