When you log on to your vcentre server you see that one of you hosts is classed as disconnected.You will find that you can still see vm`s on that host however you cannot manage any of the hosts options.
This can be resolved by restarting the management agents on the ESXI host. To do so follow the instructions below.
First press F2 to customize the ESXI host
Login as the “root” user with your password
Then go to Restart Management Agents
Then finally F11 to restart the agents
To return to the main options menu hit enter.
You will now be able to see the host again in vcentre. If it does not you can right click the host and remove it from the cluster. Then simply add it in again.