There are several possible causes for this issue.check the following step by step:
- Confirm that the vCenter Web Management Service is running.
- To resolve this issue, connect to vCenter locally with a vSphere Client located on the vCenter Server and follow Stopping, starting, or restarting vCenter services.
- Ensure that the correct DNS settings and IP address are being used
- Disable any third party web services that may be interfering with the vCenter Web Management Services.(port 8443)
- Check if vCenter Server is using custom SSL certificates as a result of a recent upgrade to vCenter Server 4.0 Update 1.
- Disable the proxy settings from the browser.
Ref: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1014454
