Fatal Error: Please Help!

Everything was working fine and all of a sudden I got this:

Fatal error: Cannot access empty property in /home4/public_html/wp-includes/user.php on line 538

which is referencing this line:

$auth_cookie = AUTH_COOKIE;

I can’t load a single page on my site now. What have I done? But more importantly, who do I fix it?

Please help!