It seems like the problem accured already yesterday and had nothing to do with the header.php work on.
+ when i change the theme to another one, title and price appear again in the preview. (e.g. xstore parent)
=> so we thought its the child-themes fault.
but, when i overwrite the xstore child theme folder with an old xstore child theme folder (over 1 week ago), the problem stays the same.
=> so it seems like the problem is not directly linked to the themes.
Please check Theme Options > E-commerce > Products Page Layout > Product content elements settings http://prntscr.com/j6a404.
Also, provide us with temporary wp-admin access.
Have you tried to disable all 3-rd party plugins?