I tried to Storage vMotion a VM and noticed that the vCetner and ESXI hosts had suddenly became unlicensed in vsphere 6.0. Looking in the licensing section the following error appeared
"the given key was not present in the dictionary"
The manage vsphere licenses was also greyed out.
Alternativey I tried to log in to the Web Client to diagnose. The following error appeared upon login
It appeared that the time had drifted on the Windows based vCenter server away from the PSC/SSO server. I adjusted this time then everything came back to life.