Problem in admin login, showing blank page in drupal 7?
After RND I found that "WSOD" or "Page not found" error occurred due to two reason.


Either Clean URL  is disable : If we want to check that Clean URL is enable or not then in URL "/?q=user/login" add this, if page shown it mean that Clean URL is disable.
Cache not clear : If your login page is working properly then login through admin &amp; clear Cache.


NOTE : After work on admin part try to log-out a/c. Sometimes due to this reason some unconditionally error occured.