Everyone knows this problem.. you move your music files to another folder, drive or subfolder.
now you can reindex the whole library but all playlist entries are broken.
my suggestion: indexing should create a checksum of each file and this should be connected to the playlist, so you can recover the broken links.
for backwards compatibility there should be a function to search for filename and/or folder etc. to find the correct link to the hdd.
You can try foo_playlist_revive https://hydrogenaud.io/index.php/topic,73910.0.html. Install the component and then, in the main menu -> Edit -> Revive Dead Items. Good luck.