How to restore deleted files after installing macOS Monterey in external hard drive

Your "My Passport - Data" volume is just not automatically showing up on the Finder's sidebar, but your screenshot of Disk Utility shows the volume is mounted at "/Volumes/My Passport - Data". So navigate to "/Volumes" in the Finder to see your "My Passport - Data" volume.

In the first place, be reassured that your files have not been deleted, otherwise you would not see "24.69 GB used" in DU.

Second, have you checked your Finder preferences to see what it should show? Yours will much more up to date than mine, but I can select items both in General and in Sidebar which determine what Finder will show me.

Third, if you know the name of one of your missing files, use a utility such as Find Any File to search for it on the external drive and see if it comes up - that way you will at least know the file still exists even if you can't currently see it.