Lost access to our custom CSS?

We have a client site that is mostly built. Suddenly, I can no longer access the custom CSS we have written. When I go to Customizer I just see the “You are customizing [client-name]” at top left and the word Widgets below. Everything else is missing. I have looked for ways to manually find the CSS file, including looking in File Manager, but no luck. In fact, the web page source code suggests that our custom CSS is being injected somehow.
Within the Elementor page editor, the Custom CSS option on the left says “Go Pro” although we have a license. I have done a long list of diagnostic steps. The only one that restored the full Customizer was changing the theme to WordPress 2020.
I submitted two tickets on this but no response.