After upgrade from version 5.2.x to 6.0 or 6.0.1 the VC may show up as being disconnected. Also a fresh install of 6.0 or 6.0.1 will cause this issue also on vCenter 5.1
The cause of this is still under investigation
The work around for this would be to perform progressive upgrades see below.
5.2.3-5.3.1
5.3.1-5.4
5.4-5.5
5.5-6.0
6.0-6.0.1