This will show you how to force iTunes to re-scan your library to fix missing metadata by re-scanning all your music files. You can re-fresh individual files by playing them in iTunes. If you have used an external program to change data and you don’t want to have to play each song to get it to update then this post will show you how.

How to force iTunes to re-scan your library to fix missing metadata and id3 tags:

• open iTunes
• file > add to library
• pick the location of your iTunes library
• (eg the ‘iTunes’ folder)

iTunes will then re-add all the files you already have to you library. iTunes wont create duplicates. Rather, it will re-scan each file and will alter the metadata for the existing entries in your library. This process can take a very long time – when I last did it I had to re-start the process multiple times for it to eventually complete. When you re-start the importing, the import starts from the very beginning again :(

If the above doesn’t work, then some people have suggested deleting your library file, and creating a new one;
this can be very time consuming if you have a lot of music –
the basic steps are

• quit itunes
• navigate to your library folder (usually Music > iTunes folder)
• delete the .itdb files (don’t delete the music files themselves)
• open itunes
• scan for new music
• iTunes will then scan your machine and re-add all the music it finds

Sources: https://discussions.apple.com/thread/2132434?tstart=0

Update: Feb 2019 (Thanks Vladimir)

This may help those with LARGE libraries of music and no better alternative than iTunes to get their music onto iPhones.
My collection is large enough that I’ve split /Music/mp3/ into 27 subfolders, 0-9, A, B, C, D … thru Z. Each new rip goes in the appropriate AlphaNum folder and that way Bruno Mars gets filed with the M artists even though I’ve tagged all the tracks Bruno Mars–keeping Picard, MusicBrainz, and CDDB happy.
The grief I found with iTunes is that the collection simply didn’t import all the way if I selected Music/mp3 as the folder to import. I’d get the A, B, and some of the C artists, if that. The fix seems to be SELECT ALL 27 subfolders (inside the Music/mp3/ location). This seemed to help iTunes by breaking up the task into 27 smaller chunks.
(I tried to just locate the music in Music/iTunes/iTunesMedia/AutomaticallyAddedToiTunes, but iTunes did some strange things to this folder even though I’ve told it NOT to manage my music for me. This folder seems to have some strange permissions at play, preventing me from even copying some files into that folder. Too bad, that would’ve been the most elegant solution…)