I have a theme, lets call it Master. I manually created a child theme, lets call in Master-Child and added some files into it. All worked well until a notification came that I should update the Master because of a new version. The child theme has failed to work since. I get an error message "Master theme does not exist" although "Master" is activated. I tried to rename, capitalize and lowercase all characters. Thanks a lot.
The only string to use the master is
Template: twentyten in the style.css if child theme.
Check, if this string correct or has the master change this. If you activate the master as Theme in your install, you find the string also in the database of the WP install, in table options, entry
stylesheet; if you have active the child theme, must have the entry
stylesheet your child theme name; the Master is always on
template. (for fast check this use the plugin Adminer)
example screenshot with the Theme "Black Label" as Master, no Child theme used. The Screenshot was in Adminer inside the WP install.