It sounds like the white screen of death probably has nothing to do with your functions.php file. I would first disable any and all caching (plugins, server-side, CDN, etc.) and dump full browser history to make sure you’re seeing the current version. Then to troubleshoot an issue like this, activate a different theme. If you clear caches again and still get the problem, you’ve verified something is wrong in the child theme.
If you verify the child theme is indeed the source of the problem, try this: save a backup somewhere, and then on the server, delete everything in the child theme except style.css. That way WP will still recognize it as a child theme, but you can then add files back into the child theme one by one – clear caches, revisit the site, each time you’ve added one more file – and in this way you can isolate for sure the file that is causing the issue.
Once you know which file is the problem it should become easier to figure out what the specific problem is.
If you find that your child theme has nothing to do with the issue, it could have to do with any number of other things. For instance, a security release of WP Core just came out – perhaps it contained something that conflicts with one of your plugins or your server setup. So if the child theme isn’t the culprit, the next thing to do is to deactivate all of your plugins – easiest way to do this is to simply rename the plugins folder on the server, and WP will deactivate them all. Then clear cache and test the site again. If this fixes your issue, rename plugins back to its normal name, and activate one plugin at a time, clear cache, test the site, until you find which specific plugin is having problems.