If you used the Play Music app to select music for offline use, that's encrypted and authorized only for a single account. When you switched accounts, that offline library will be gone because for Play Music, that account is no longer authorized (unless you download things again for that account).
Now, your music is not lost. It should still be safe in the cloud, it's just the offline copies that are affected.
If you have purchased music on Google Play Music or have uploaded your own and you want to use that for several accounts, you'd have to download the actual MP3 files from Google Play and copy them to your phone. Unfortunately, this can only be done via a web browser (either on PC or forcing Desktop Mode on your phone's browser). Music that you don't own cannot be downloaded this way, but it can be pinned for offline use if you have a Play Music Unlimited or YouTube Red subscription.