My Additional CSS is Missing!

What happened to my Additional CSS? I’ve been using the Child Theme of my theme and had saved years of customization codes in my Additional CSS section of the theme and now they’re all gone. How can this be? My site is still reflecting all of the additional CSS codes. I’ve implemented over the years and nothing is missing but now I’m scared to update the theme without the assurance that my additional CSS codes are somewhere. I did install a new plugin name WP SMS. Any suggestions? or can you tell me if I updated the core theme that it won’t affect my site since I’m using the Child theme?