have you been changing anything in the folders via ftp?
i'm not sure exactly why it did it, but i had the same thing happen to me a few times on one of my blogs quite a while ago. it turne dout that any time i was altering files and reuploading new files in the theme through ftp that it would reset the theme on me. seemed strange, but it was totally what was doing it to me.
not sure what to say could be your problem though, sorry
