I'm unclear as to what you mean by "offline". AltaVault status is reported as a healthy (green checkbox), degraded, or critical state (red X). If you're referring to AltaVault being offline from a Backup Exec 2015 perspective, I've seen this problem surface if the AltaVault is not able to be reached due to a transient connection problem over the LAN, or if the AltaVault service is down in critical state when Backup Exec is attempting to begin an operation to it.
You should be able to correct the problem by reconnecting to the AltaVault storage target from Backup Exec (reviewing the storage device properties, as an example). You may also have to add the AltaVault hostname/IP pair to the local Windows HOSTS file as well. I believe there was an older technote from Symantec (pre-Veritas) about this requirement for BE2015 to recover from this type of condition, although I can't recall what number it was.