The most straight forward way to troubleshoot is checking the log files. This could e.g. be caused by an issue with the storage system (less likely) or maybe by a Windows host with access to the LUN. If a Windows system with "Automount" enabled has write access to a VMFS LUN it may change the partition type to NTFS. Depending on the ESXi/VMFS version you are using, you may use either fdisk or partedUtil to check the VMFS datastores.
André