Virtual machines show in VMware VirtualCenter/vCenter Server as Unknown VM.
When an ESXi/ESX host is rebooted or a host agent is restarted, it needs to reload the host agent configuration of each registered virtual machine (.vmx file). If the .vmx file is inaccessible, the ESXi/ESX host is unable to read the configured name of the virtual machine, and it defaults to Unknown VM.
Temporarily losing access to storage does not cause a virtual machine’s name to be permanently set to Unknown VM. For more information, see Identifying Fibre Channel, iSCSI, and NFS storage issues on ESX/ESXi hosts (1003659).
When storage problems are addressed, restart the management agents on this host. For more information, see Restarting the Management agents on an ESXi or ESX host (1003490).
If the virtual machine is still renamed as Unknown VM, examine the /etc/vmware/hostd/vmInventory.xml file on the host showing which virtual machines were formerly registered. Make a note of the virtual machines and the datastores on which they are located and then re-register the virtual machines.
To re-register a virtual machine:
- Right-click the Unknown VM entry and click Remove From Inventory.
- Browse to the appropriate datastore for the virtual machine and open the folder.
- Right-click the *.vmx file and click Add to Inventory.
- Power on the virtual machine.
- Repeat Steps 1 to 4 for all unknown virtual machines.
Source : https://kb.vmware.com/s/article/2172