You really need to provide some details.
We do not even know wether the whole volume now apparently is fat32 formatted or if there is only a vmdk that is fat32 formatted.
Thats why we need to know the partitiontable of the disk that contains the VMFS-volume.
If you want to know wether this can be reverted we need to see the partitiontable AND the output of the command:
dd if=/dev/disks<vmfs-partition> bs=1M count=2 | hexdump -C > /tmp/vmfs-start.bin
Given the data we have now we have to assume the reason for this is either a silly user mistake or sabotage.