Widget and Customize menu not working.

This may be a plugin or theme conflict. Please attempt to disable all plugins, and switch to the default Twenty Seventeen theme. If the problem goes away, enable them one by one to identify the source of the problem.

If you can install plugins, install Health Check. On the troubleshooting tab, you can click the button to disable all plugins and change the theme for you, while you’re still logged in, without affecting normal visitors to your site.