Unable to Customize Theme

For some reason, I no longer have the ability to customize the theme.

Anyway I try to navigate to the customize screen results in a blank page where the preview should be. The left navigation bar says “You Are Customizing Missoula Mavericks” but none of the options are displaying.

I am building a new website: missoulamavericks.com

I did some research and really like GeneratePress.

I have been customizing the theme as needed, but now the Customize page is only showing a blank page as described above.

Please help as I need to customize how the footer widget displays on the posts page and am unable to make any changes.