On Windows, the ProgramData folder is hidden by default. If you tried to manually move files, you might have accidentally broken the file permissions. Ensure you are logged in as an Administrator.
If you are on a Mac, especially with newer OS versions (Monterey, Ventura, Sonoma), your DAW might not have permission to "see" external drives or certain system folders. ezdrummer 3 no midi library found
Once your MIDI library is back, take five minutes to prevent this nightmare from recurring: On Windows, the ProgramData folder is hidden by default