Customize not available

I have several sites using Customizr. On one site I am unable to customize the theme. This is fairly new, have not made any changes to the site. When I click on Appearance and Customize it simply says Loading… and then the menus never show up. Done all the usual things but this doesn’t seem to work. My site is up but I now have no idea how to get into the area I need to to make changes.