As Tom noted, it’s never safe to edit WP Core files, including any of the contents of wp-admin or wp-includes.
What happened? Your site may have auto-updated, and any update to Core would have overwritten your changes. Or, even if your site didn’t auto-update, it could be that your host has recently locked down those folders so you can’t just access the files directly.
How to proceed? Check your own backups, or if you don’t have one, check with your host. Many hosts offer backups for a fee, even if you pay them after the date you want to pull backups from – i.e. many of them are already storing backups and just not making them available until you pay. Once you have those backups, it may be quite a process to go through all the files, compare them to the Core versions, and pull all that functionality out into separate plugin files so that your code is safe and doesn’t get overwritten when Core is updated.