I have been working on a Drupal 7 install with the Bartik theme. Today, the main menu tabs changed to a vertical list on the left side of the page. I do not know how to reset this. I checked the theme folder and none of the css files have been changed, so I suspect it is either a module I installed today or something I did when I was working on editing some pages. Any suggestions would be aprpeciated.
Try turning off "Aggregate and compress CSS files" on /admin/config/development/performance and then use right-click inspect element (firefox or chrome) to identify the css responsible for the override. You should see the bartik css files overridden by something else causing this behavior.
A clear cache never hurts either.