Sometimes a grey exclamation point will appear next to songs in iTunes, indicating that iTunes cannot locate the file. How does this occur and how can you fix it?
Inside the iTunes folder, there is a file called "iTunes Library" that tells iTunes where the "iTunes Media" folder is located. (This is the folder that actually contains your music, videos, apps, etc.) When iTunes tries to access content (e.g., you try to play a song) it gets the location where the song is stored from the "iTunes Library" file and then looks for it in that location. If it can't find the file, it displays a grey exclamation point to the left of the entry in iTunes.
This can occur for a variety of reasons. iTunes does not dynamically keep track of the location of your media, so if you move your "iTunes Media" folder or anything within it from its original location, iTunes will be unable to find it. Renaming anything inside this folder is also verboten. I found this out the hard way when I changed the name of the folder containing songs by Sinead O'Conner (I didn't like how iTunes chose to spell it). Sure enough, every song had an exclamation point next to it because it could no longer find any of the tracks. The moral of this story: manage iTunes content via iTunes, not the file system. Sometimes it's not clear why iTunes can't find your songs—it's possible that the index has become corrupt.