I had this last week - thought I'd locked myself out permanently - but I deleted my cookies using the browser and this allowed the new one to be added when I logged in and then I was able to change settings back and everything was ok/
This prompted a question though - which I have posted elsewhere here... My users and I are having to log in twice for the log in to stick... I am using the default cookie settings and am now afraid to change them to my domain and root path incase everyone gets the impression they are locked out and cannot get back in. So I posted about problems with logging in to tell users to delete their cookies and try again - but people still have to log in twice! Hmmm.