Admin just… stopped working

I had this problem with a previous version of WordPress, so I updated to the latest version, and I’m still having the problem. The blog was set up, I was able to post, and everything was working great. Then the admin just disappeared. I click on admin or go to wp-login.php, and there’s nothing that shows up. It’s completely blank. I view the source code and there’s nothing there. Any idea what could be causing this problem?