Hi Michael,
Thanks for reaching out and thank you for using Crio WordPress theme! Have you seen any error messages associated with the customizer not loading? You can try inspecting the your browser console for JS error messages, it might reveal something useful. You can copy/paste any error messages you find into the thread here for us to inspect.
A similar issue was brought to our attention in a previous contact and we were able to confirm that the issue seemed to be related to the menus used in the custom header/footer templates.
When you add a menu to a custom page header or footer it needs to be registered to the WordPress menu system and have a menu assigned to it. Each new menu that you include should have a unique name, as in you can’t have two menus registered to the same custom header/footer template that also use the same menu location or it will cause the customizer to break.
To resolve the issue trying moving all custom headers/footers to the trash to see if this resolves your issue. Similar to deactivating plugins but once you find the problematic template you’ll either need to correct the issue in the template or recreate it entirely.
I hope this makes sense Michael. If you ever have any other questions for us in the future please do not hesitate to ask!