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 template and 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.
