wp-admin / wp-login.php is simply taking me to 404 error page

There is a strange WordPress login problem. When i hit mydomainname.com/wp-admin, it’s simply taking me to 404 error page and if i try with mydomainname.com/wp-login.php, its showing the login page. But upon giving the credentials, its showing 404 page again.

Have changed credintials to 707 but didn’t work. i cant access my dashboard too. Pls provide solution.

Rgds,