Can’t acacess any admin screen

I’ve got a really odd problem after a failed upgrade last night. I was upgrading to the newest version with the WordPress Auto Upgrade plugin. I have used that plugin in the past with the site without issue. Last night it completed the preliminary checks and reported all was ok, and then when I clicked to begin upgrade it threw me back to the index page. And now I can not get to any admin page. Clicking on the login link or trying to access any admin URL loads the normal index page. For example, if I click on the default log-in link on the theme it should load /blog/wp-login.php. What I get in the address window is /blog/wp-login.php/ and the main index page is loaded. Obviously, there is no directory named “wp-login.php”.

Stuff I tried so far…

1. Complete upgrade via ftp. It made no difference
2. I’ve checked for rogue .htaccess files that could be redirecting – don’t see anything
3. Checked wp-config.php – it looks fine.
4. Checked dB options table for unusual entries – it all looks fine.
5. Chekced theme directory – no files were changed yesterday.

Any idea what has happened and how I can fix it? I’d rather not blow away the site and start over with a clean install.