Are you using the jetpack plugin? I’ve seen someone in the past using it, and they said they were unable to access their stylesheet when it was activated. Not sure if that’s what your issue is, but it’s the only thing I can think of.
I had it installed on this particular site and deactivated it, but it is still not showing up. I have Jetpack installed on another site using Epik and I can see the style.css file in the Editor.
ellaj – who is your site hosted with? There are a number of hosting companies who have disabled access to the editor functionality in Appearance > Editor.