Hard to say without knowing what happened. Anyway, I'd start checking whether the LUN still shows a datastore/partition with type "fb" (VMFS). Maybe another OS (e.g. Windows) had access to the LUN and changed the partition type to "07" (NTFS)!? To check this, run fdisk -lu from one of the hosts' command line.
If you are unsure, I'd strongly recommend that you consider opening a support case with VMware!
André