Is your XenCenter an older version than the XenServer Host you are attempting to connect to?

The XenCenter application is backward-compatible and can communicate properly with older XenServer Hosts, but an older XenCenter cannot communicate properly with newer XenServer Hosts.

To correct this issue, install a XenCenter version that is the same, or newer, than the XenServer Host version.

Is your license current?

You can see the expiration date for your License Key in the XenServer Host Overview tab under the Licenses node.

Also, if you upgraded your software from version 3.2.0 to the current version, you should also have received and applied a new License file.

The XenServer Host talks to XenCenter via HTTPS over port 443 (a two-way connection for commands and responses using the XenAPI), and 5900 for graphical VNC connections with paravirtual Linux VMs. If you have a firewall enabled between the XenServer Host and the machine running the client software, make sure that it allows traffic from these ports.