Admin User not able to Login

I have a wordpress website. http://www.bullrider.in , till last month it was working fine, but recently I have started facing a problem due to which I am not able to LOGIN to wp-admin using my administrative Id, whereas authors are able to login.
I see the login window, when I enter my admin user and password the page is redirected to a Page Not Found 404 page.
I have tried the following things but nothing has worked so far :-
File Permission
Plugin Disabling
htacess modification
creating a new admin using MYSQL ( but then also same problem was there)
None of the above mentioned have solved my problem, please help me
thnx:)