Can't logon to the vCenter Server

Just had a strange problem.  Been able to logon to our vCenter server without any problem.  All of a sudden I am getting this message

Call "ServiceDirectory.QueryServiceEndpointList" for object "ServiceDirectory" on vCenter Server "<vCenter servername>" failed.

As far  as I can tell nothing has changed since the last time I logged into it.  Have done a google search and looked at the VMware KB's but cannot find this error anywhere.

Anyone else come across this?

Thanks