Years ago, probably under a different Apple ID (email changed), I uploaded my entire CD library to iTunes. When I log in now the only music in iTunes is that which is on a connected apple device. Is there a way to repopulate/restore my old library of music?
Not unless you have the media on a computer, device, or backup it can be restored from. Signing into your Apple ID on a new computer gives you access to your unhidden purchase history, not everything you might have loaded into iTunes on a previous computer, unless perhaps you've had an ongoing subscription to Apple Music or iTunes Match. A new ID won't have the content of an old ID.
See Recover your iTunes library from your iPod or iOS device - Apple Community if there is media on the device that isn't in your library.
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Missing Music in my iTunes Library
