Unable to open SharePonint 2010 site in SharePoint Designer 2010 even I have full control access.

Unable to open SharePonint 2010 site in SharePoint Designer 2010 even I have full control access.

i am getting below error as popup.

"Unable to open web site. Possible causes:

1. The web server may not have SharePoint server installed
2. The web server may be temporarily out of service.
3. If you are through a proxy server, the proxy settings may be incorrect.
4. An error may have occurred in the web server."