Switching between version 2.0 and version 3.0 (in either direction) is known to require a manual database rebuild.
If that's what you did, and you didn't manually rebuild the databases after changing the version, then then having it get stuck on "rebuilding music databases" is normal, and you just need to do the procedure that Doug linked.
If you're getting the stuck-on-database-rebuild thing on a system which did not switch between 2.0 and 3.0, then something else has gone wrong. Try rebuilding anyway, but keep an eye out for other errors.