Upgrade 2.3 problem

I can’t click on my comments or view a page in a post after I have upgraded.

The page isn’t redirecting properly

Firefox has detected that the server is redirecting the request for this address in a way that will never complete.

I have deleted all old files except for the ones i was told not to delete. I also have had problems updating the .php files. I recieve the following error message no matter what theme I am using.

The requested theme does not exist.

HELP!!!