Having only "New" choice item in the "Style Sheets:" list

Using Alter CSS with the Garland theme, it seems I made two mistakes. If I remember well, I first changed the name of the style_backup.css from within the Theme administration page with the "Save CSS file button", and then erased the renamed file by ftp. The result is I now only have the "New" choice item in the "Style Sheets:" list, even though the style.css is still in the folder, and even if I try to create a new style_backup.css file or a css file with the renamed name. I tried deactivating, erasing and re-uploading the module, but with the same result. Any suggestion as to what could bring back this functionnality?