This just happed to me. Signing in and out of iTunes didn't fix it for me. It took a few restarts of iTunes, and I had to turn iCloud music library on twice.
iTunes is scanning my library now and has put a cloud icon next to my each of my 32k tracks. It's going to be uploading and matching for a week, at least.
It was working fine 3 days ago, and there wasn't a lapse in my Apple Match subscription. The only thing that is out of the ordinary is that switched from Apple music to Apple Match 4 weeks ago. I didn't realize how much of my personal music collection wasn't in iTunes and re-added thousands of songs to my library. Maybe it was the heavy work load that caused iTunes to flip out