Inaccessible Virtual Machines:
If your virtual machines are inaccessible, that means that the vSphere client is not able to reach the *.vmx file.
Check in your storage if your datastore is available.
Unable to add datastore:
If your VMFS store is missing and are trying to add VMFS store through vSphere client through storage configuration and is the datastore still does not appear.
Log in to ESXi host using shell or putty and run the following command
#esxcfg-volume -l
This should show the volume.
To persistently mount the volume, use
#esxcfg-volume -M followed by the name of the volume
e.g.
#esxcfg-volume -M 4e3f2ad1-a3d403b1-ddaa-8d2b2e73ad10