Cannot Customize Header

I cannot access the header customiaation page. I go to ‘Appearance’ then click on ‘header’ and I get a virtually blank page with ‘You are Customizing ‘website name’ and the Save and Publish button but nothing else. All else is blank with little whirling circle in the center and a ‘customize: loading’ message permanently showing.

This happened immediately after I uploaded a jpeg file to use as the header in the Eleven40 Pro theme. I selected the new header from ‘media’ files, and clicked to Save and Publish. I checked how it looked and it did not completely show up, being cut off at the right hand side of the logo/text header, so went back to change the dimensions and couldn’t get back in.

Pete