Error Messages 101 and 500

Hi,

I was trying to update my site a couple of days ago and it suddenly started with the error messages 101 when I tried to log out (or just a simple – Access Denied), and a Error 500 message when I try to access various parts of the editing board, such as media. I cannot delete blog posts. The Error 500 message comes and goes but I have not been able to log out since then. Can someone please help?

Thanks