Why Did My Website Change?
When you apply your child theme you may have noticed a few areas of your website are different. This is because WordPress saves these customizations on a per-theme basis.
When a child theme is activated you have technically switched themes according to WordPress. Because of this, you will need to reset your menu locations in "Appearance > Menus", add your customizations back to "Appearance > Customize" and potentially save your permalinks in "Settings > Permalinks"