It looks like you may have a plugin which regulates your Joomla admin which would cause this, such as AdminExile. I recommend taking a look through your plugins directory inside your Joomla folder and look for recently modified files and folders. From there, you may begin renaming various plugin folders to determine the culprit.
Jeff I am the admin and what actually happened was that I wanted to install a new version of jomsocial but the requirement was for joomla 2.5.8 but at the moment I had a lesser one so I did the manual installation by going through the extensions manager, and then I succesfully installed the jomsocial. But i tried loggin in today and I couldnt login
From taking a look, it does indeed appear to be an issue with a plugin that interacts with your administration. Unfortunately, I cannot discuss specifics of your account via this public forum, but you may contact technical support for further information on this.