Recently upgraded one of our HP servers to VMware vSphere 4.0, and found that we couldn’t see the hardware status from the vCenter client.  All it would print was “Hardware monitoring service not responding, the host is not powered on”, despite the host certainly being powered on.  A VMware Communities posting suggested that it was either firewall related (nope), or that removing the host and re-adding it might work.  Turns out there was a third option – just disconnect and reconnect the ESX host.

So simple, and now I have full visibility of the fans and temperatures on the host server.