Presentation Broken by plug-in

I just installed and activated a new plug-in, and then when I go to Presentation, instead of getting the nice list of templates, it actually lists the contents of the css file itself.

After deactivating and removing th plug-in, it is still behaving strangely.

The front-end (the blog itself) looks fine. This is the second time I’ve had this problem, and the first time I had to resort to re-uploading the whold website. Just re-uploading the template files doesn’t help.