If you take a look at the hardware status in your vCenter 5.x and you see the following error…
No new host data available. Data will be updated in 5 minutes
…you can try to click “Update”. If some seconds later the following message appeares, there may be a problem with the “vCenter Inventory Service” in most cases:
Host sensor data is not updated. The Query Service is not available or was restarted, please retry.
To fix this problem log-in to your vCenter Server and go to “Services”. Check if the vCenter Inventory Service is started.
If it is running, restart the “VMware VirtualCenter Server” Service. Thereby the “vCenter Inventory Service” will be restarted, too. In most cases the problem should be fixed.