Problem in admin login, showing blank page in drupal 7? Problem in login admin part.

Whenever I try to click on login link or try to login directly from homepage. It shows white screen. Which type of problem is this. I can't see the error logs due to problem in admin login. 

Is this issue due to low server space ?
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.