When I logged in and check with Farm Admin account, even I was unable to add, edit, or select any menu item. So I quickly navigated to Central Administration Site 2010 and accessed the Site Collection Quotas and Locks under Application Management Category.
I chose my site collection and found that in Site Lock Information, the option was already enabled to "Not Locked". Hence I simply clicked on "No access" and then again clicked on "Not locked" and finally press OK button. Thereafter I checked my site collection and everything was back on the track.