Hey there,
Sound like you have not backed up your Customizer settings. Also, switching to the child theme will reset your widgets and menu so you need to assign them to your widget areas again. For the menu, you need to assign the Primary menu.
Please switch to the parent theme again and backup the Customizer settings. See https://community.theme.co/kb/customizer-manager/ for more details. Then, take note of your widgets and menu setup. After that, activate the child theme and import the Customizer settings.
Thanks.