Music keeps quitting unexpectedly when I open it

Restart your computer with the option to reopen closed windows turned off.

Press and hold down option+cmd as you launch Music to open it in its safe mode which can sometimes help with odd behaviour.

tt2

Does holding down option as you launch Music allow you choose or create a new library? Music cannot be downloaded independently, but you might be able to restore the app from Time Machine, if it is set up to include the applications folder. Alternatively you might need to reinstall the entire OS.

tt2