Are you using 5.5 or 6.0? If you are still using 5.5, see if the thick client works. If 6.0, tail the C:\ProgramData\VMware\VMware VirtualCenter\Logs (Check the most recent log file for vpxd-xxxx).
I've had this problem in the past and it is usually the SSO server which is jacked or an issue with the VMware VirtualCenter Management Webserver. Try restarting that service and tailing the logfiles as well.
Post back with the results, even a snipit of the logs should be helpful Good luck!
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