Hopefully it's not been too long since you last upgraded iTunes, in fact if you get an empty/incomplete library immediately after upgrading then with the following steps you shouldn't lose a thing or need to do any further housekeeping. In the Previous iTunes Libraries folder should be a number of dated iTunes Library files. Take the most recent of these and copy it into the iTunes folder. Rename iTunes Library(.itl) as iTunes Library Corrupt(.itl) and then rename the restored file as iTunes Library(.itl) then start iTunes. Should all be good, bar any recent additions to or deletions from your library. Newer Mac & Windows libraries use the .itl extension. A library initially created on an older iTunes for Mac version might not.