Some details would be helpful, e.g.
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.