Theme just disappeared from site

I have a site I'm working on that uses a custom theme modified from Zen Basic. I was working on the site tonight in CiviCRM, and all of a sudden, it was like my theme just disappeared. Any page I went to, all I got was text on a white screen. I logged out, and all that was displayed was the front page text. I had to go to mysite.com/user to log back in, and change the theme to something else. The weird thing is, all my theme files are there, but it's as if they don't exist. On the Admin -> Themes page, my theme isn't even listed any more. I can change to other themes just fine. Bit when I look in sites/all/themes directory on the server, all the theme files are there just as they always have been. I double checked under the themes/engines directory, and everything is there, too. What would cause a theme to be completely ignored all of a sudden like that?

Thanks.