If the hosts work and it's only the Virtual Center Server which doesn't show the correct state, it may be an option to check the log files to see whether there's some kind of connection issue (DNS resolution, certificates, ...). You may also want to check the licensing state on the hosts.
If you cannot find a specific reason, restarting the Management Agents on the host might help in this case (see http://kb.vmware.com/kb/1003490).
André