I created a custom theme awhile back, and everything has worked fine up until now. All of a sudden, my theme was reverted back to the default twenty eleven theme because apparently my custom theme is missing it's stylesheet, but the stylesheet is clearly there. I link to it correctly and everything. I don't understand why all of a sudden it would stop working? Any suggestions?
Have you changed anything in the stylesheet? WordPress looks for the header of that file to be in a specific format. http://codex.wordpress.org/Theme_Development#Theme_Stylesheet
You could also check that all folders/filenames are consistent. Hopes this helps in some way - good luck!