VM data is lost all of sudden.

Check the VM config to make sure the issue is not related to an allocation of an additional disk from the new, deleted LUN that prevents the system from booting.  It is hard to understand besides that what might have gone wrong. besides creating a new LUn what else was done? i.e. moving .......
Check the references in the vmdk files.... are you able to see the VM parameters settings?

Has a LUN been removed, or deleted which a VM was stored on ?

-->It is hard to understand besides that what might have gone wrong. besides creating a new LUn what else was done?
Added a new LUN and deleted/

-->Has a LUN been removed, or deleted which a VM was stored on ?
No. He tried to add a LUN and deleted that LUN for testing which was not in use in the SAN

As per my understanding his VM was using one of the LUN which is not listed in the inventory. I was told that he got an warning that HDD will be formatted and all data will be lost. Do you think it can happen?

Unless he deleted the wrong LUN, no changes should be impacting the VM.

As per my understanding we cant delete a LUN if it is in use.
I don't know what he did?
I checked his previous config and new config. I cant see any changes to the old datastores which is added by me when installed.

check the VM on what the issue is, you might have to see whether an update went wrong and reverting an earlier snapshot might resolve the issue.  What is the error that is displayed...
without walking through the exact steps that were taken that might/may have had some ......

Cant even boot to OS and cant see any snapshot either. I connected this HDD to another server to see the data. It is like a newly created HDD.

Please look at the VM config, where are the DISK that are allocated to it, or was it always booting from the network?
To fix the issue, one must identify what causes the issue. Whether it is missing the VMDK, or there is another change where the DHCP, BOOTP are no longer functioning for a VM that boots and loads data from the SAN....i.e. diskless client...

You can delete a LUN if it's in use on the SAN (e.g. delete it on the SAN!), and it will cause no end of issues, as it disappears from hosts and everything on it, causing what you have screenshotted, as the disks have been ripped out from underneath the OS.

if there is no data in the VMDK you have connected to another VM the data is corrupted or gone, a restore would be required.

Can you check the location of this VMDK, it's on a SAN datastore which has not changed ?

as for this - Huawei Oceanstore 2200 - is this Certified for use with ESXi ? I assume it's the 2200 V3 ?

-->Please look at the VM config, where are the DISK that are allocated to it, or was it always booting from the network?
I can see both HDD with same size but no data. DATASTORE vMotion is the name of datastore.

Are these RDMs ? e.g. RAW LUNs on the SAN ?

That looks a bit odd, a RAW LUN as a BOOT OS disk ?

DatastoreVMotion still exist ?

-->if there is no data in the VMDK you have connected to another VM the data is corrupted or gone, a restore would be required.
He doesn't have backup for this VM. :(

Can the client remember HOW this VM was provisioned ?

RDM or Virtual Machine disks e.g. VMDKs stored on a datastore.

Do they know what a RDM is, and why were they using a RDM for what purpose in the VM, because it is highly unusual to use a RDM for C: and D: in a VM....

because it looks like a LUN was destroyed..... or missing... and the VM config has just got screwed up, because all the VMDKs are GONE, because the LUN is missing.

-->Can the client remember HOW this VM was provisioned ?
I checked other VMs in the same host that VM also displays same Mapped RAW LUN

-->So they are using RDM for all VMs ? - OKAY.
I saw 90% of the VMs using RDM.

-->Thet would need to check documentation to work out which LUNs on the SAN match these RDMs.
How to check this?
It will be a great help if you can guide.

-->Contacting the SAN vendor may prove helpful in an attempt to recover the LUN if possible...
He didnt delete the LUN from SAN he added the LUN which was not added b4 and added as a new LUN in ESXi.

How to check this?
It will be a great help if you can guide.

what they wrote down! in their recorded documentation. If you use RDMs, you have to have a recorded LUN ---- VM relationship to know, what LUN on the SAN is what disk on the VM!

otherwise the only way to investigate further is to look at all logs of ESXi, vCenter Server and SAN, but I'm not sure they will get you anywhere, someone has made an error.

Thanks