If you have migrated your Nextcloud instance to a new system, or simply added files over FTP/SSH instead of the integrated uploader, you can use a command to recreate your file list. Navigate with a console (e.g. a SSH terminal) to your Nextcloud root folder, then execute the following command:
sudo -u www-data php occ files:scan -all
This will recreate the file list for all accounts on that instance.