Profil & Dasboard Blank Page

Hi, I have an interesting problem.

This occured once only for a super user account before. So i rebuilded user account and fixed the problem. But today the problem suddenly occured again and for all users. I cant open dasboard page. only a blank page appears. I check blank pages' html source codes, only there is

 <a href="/  

i did not made any changes on my web site. Please take a look at my web site.