For general library squiffiness following an upgrade the easiest thing is to restore your last backup, but I guess if it were that simple you wouldn't be here.
Empty/corrupt library after upgrade/crash
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. Start iTunes. Should all be good, bar any recent additions to or deletions from your library.
See iTunes Folder Watch for a tool to catch up with any changes since the backup file was created.
When you get it all working make a backup! For example grab SyncToy 2.1, a free tool from MS. This can be used to copy your entire iTunes library (& other important data folders) onto another hard drive or network share. You can then use SyncToy periodically to synchronise or echo your library to the backup. A preview will show which files need to be updated giving you a chance to spot unexpected changes and during the run only the new or updated files will be copied saving lots of time. And if your media is all organised below the main iTunes folder then you should also be able to open the backup library on any system running the same version of iTunes regardless of drive letter or path.