how can i undo a force quit

Rebooting or restarting your Mac is an option when you would otherwise be shutting it down. See How to turn your Mac on or off - Apple Support.

See Use Time Machine to restore deleted files or older files - Apple Support. If your library has somehow become corrupt and Time Machine is working then it should be possible to restore the state of the file ~/Music/Music Library.musiclibrary from Time Machine from a point in time before things first went wrong to get it working again.

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It cannot be undone. What happens when you try to launch Music now? Is there an error message? Have you rebooted your computer? Do you have Time Machine set up and protecting your library?

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