Ubuntu Software, a.k.a. Snap Store shows empty categories (or doesn't show categories at all, only a few software picks on the front page and nothing more) when there are access errors on snapcraft.io server, which recently happens quite often. You can find the error messages in system log, or you can kill the running snap-store process and run again snap-store in terminal. If you see "access denied" errors in snap-store output, then something happened again to snapcraft.io server. This is a temporary problem that goes away when they fix the server and it starts to operate normally again.