First, be sure you are on the "All" tab to make sure you can see all your files.
Files are only stored on your own device, so if the device is lost or broken (or if you deleted/reinstalled the app), files will be lost.
Downloading Locker on a new phone will not show your items, as they are only stored on the phone that they are added to Locker on. There are no user accounts or cloud storage so they will not show up on a different phone. You must transfer your files from your old Locker to your new if you get a new device, or restore an iCloud Backup from your old phone.
If you added files to your current phone, but now they are not showing up, try "Export All Files" in the menu, this will create a zip of any files within your Locker, even if there is an issue displaying them. Note that if you ever deleted the app, that would delete the files inside.
If you cannot open the app, or files are not showing up, you can always easily export all your files to a Mac or PC, here’s how: How to export all Locker files to Mac or PC
Note that if the app was deleted or device lost/broken, the files cannot be recovered, unless your device is backed up with iCloud backup. You can check whether you have iCloud backup here: How to check if iCloud backup is enabled