I had upgraded my blog without backing it up first, something I never do. I noticed after the upgrade and the theme change that the links to the archives were missing, returning 404 error codes. I tried repairing the permalinks and repairing each database table with no luck.
Thanks to the Penguin Web Hosting folks who host my site, they rapidly compared the latest set of fileson their daily backups with the current directories and noticed the .htaccess file missing.
The file was restored and the links to the archives are functional again.
If you ever wonder what the Apache .htaccess file does, here’s a good hyperlink to a page on the subject.
Never upgrade or promote a release to your prod environment without a backup or a rollback plan.
I should have known better :-p