iTunes purchased music gone?

How did it "suddenly disappear"?

What changes did you make to your system – e.g., updating macOS – recently? Have you made any Apple-Music-subscription related changes (starting or stopping a subscription, enabling or disabling "Sync Library") recently? It is nearly impossible to troubleshoot without context.

Note that the iTunes application went away in Catalina – replaced by separate Music, TV, Books, and Podcasts applications and the Finder. When upgrading from an old Mac running, say, High Sierra, to a new Mac running, say, Tahoe or Golden Gate, it may be necessary to convert an old iTunes Library into Music and TV libraries.

Were you keeping backups? This is always a good idea so that if catastrophe happens, there is a way to recover.

If necessary, you may be able to redownload music from the iTunes Store.

• Redownload music from the iTunes Store on your Mac - Apple Support
• Redownload music from the iTunes Store on your PC - Apple Support
• Redownload music from the iTunes Store on your iPhone or iPad - Apple Support

This depends on the music still being in Apple's catalog. In some cases, the record labels or artists pull music from iTunes Store distribution, and then you can't redownload past purchases. You can only restore them from your own backups.

Likewise, you may be able to redownload books.

Redownload books and audiobooks - Apple Support