Assuming you are using ESX 4.1 (that's what I found on your other posts), run fdisk -lu from the command line (putty) to see whether the VMFS datastore's partition type is still "FB" (VMFS). If this is not the case and the partition type is e.g. "07" (NTFS), but the VMFS datastore is still accessible from the command line, it may be possible to use fdisk to change the partition type back to "FB".
André