Error while opening themes page in wordpress admin

Solution:

One of your theme file or function is missing, which cause fatal error on your server. You can check server log by enabling debug mode inside wp-config file after database credentials.

define('WP_DEBUG', true);
define('WP_DEBUG_DISPLAY', false);
define( 'WP_DEBUG_LOG', true );

Then try to access your themes tab again and see errors in wp-content/debug.log file.

Note: Disable debug mode on production or you can revoke public access to this file.