you get an error message or just doesn't play?
maybe the path to you library is broken or changed? Is you Library in the default location or on a remote drive somewhere?
go to iTunes on your computer... go to Setting > Advanced and see if the path to your iTunes directory is where you expect it to be? if it isn't change it back.
go to iTunes, right-click on a movies or song you know doesn't play and click on Get Info. If the path is changed iTunes might ask if you want to Locate file, say yes.. if you do this for 3-4 files iTunes asks if you want to check for all files.. say yes...
sometimes the path temporarily changes and goes back because of connectivity problems... so the act of pressing Get Info it actually fixes the path for that file.. so if Get Info doesn't yeild a Locate Dialog Window... try the file on AppleTV now...