Why no backup?
Why low disk space still perform MacOS upgrade?

All that aside.

You might want to try login as root user and see the "data" volume show up. It might be hidden or unmounted due to MacOS upgrade progress & permission issue. Try terminal command and check your "data" volume still exist, "diskutil list".

You might also try do a search for the user folder in finder. MacOS installer will move or archive the user data during installation.