Hi,
had the same issues and couldn’t find a solution. Media wouldn’t show thumbnails but everything was there. I actually solved this. The issue for my website was connected to standard wordpress memory of 64MB, so I changed my wp-config.
Here are very useful links to do that:
https://www.wpbeginner.com/wp-tutorials/fix-wordpress-memory-exhausted-error-increase-php-memory/
https://www.wpbeginner.com/beginners-guide/how-to-edit-wp-config-php-file-in-wordpress/
Worked like a charm.
Hope it works for you too.
