If you ever made a backup of your music, which means using the computer's File Manager to make a copy of the music files onto another drive, try using that. (Note that the backup feature in iTunes does not back up the music files. That is something that you have to do this yourself.
If you do not have a backup, then try the following:
- If you are referring to music that you purchased from the iTunes Store, have you logged into your iTunes Store account with that same ID and checked your purchase history? The article mentioned at the top of the page under Related Article should assist you with this.
- If you are referring to music that you purchased from other online sources, go to your account with that store and look for your purchase history.
- If you are referring to music that you ripped into Tunes from CDs, then you will have use that CD to re-copy the music into your library.