Before trying to log in, log out, or troubleshoot elsewhere, try checking Apple's Services, Stores, and i Cloud system status website.
If you still can't find what you're looking for, it may be time to contact Apple to see if they can directly solve your problem.
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Just be aware that your whole library may have to be scanned again and, depending upon its size, this could take hours or even days.
If simply turning i Cloud Music Library itself off and then back on doesn't seem to fix the issue, try signing out of i Tunes completely on all your devices, including your Mac or PC.
Then sign back in and try syncing your i Cloud Music Library again.