4
One quick method to try is to log back in by going to www.yoursite.com/user.php, it might or might not let you depending on the problem
If you get a "your logged in "message", go to www.yoursite.com/admin.php by typing the path in your browser if you need to.
Then go to system admin-->preferences-->general settings and switch on phpdebug, then go back to the page with the problem and copy and paste the error results here.