iTunes AirPlay option disappeared

Hi everyone,
I have a MBP (mid 2009) and I was going to stream music from iTunes to my speakers over the Airport Express like I've done a million times. The music started out fine, but then I clicked back to restart the song and the music cut out and the Airplay button disappeared from iTunes. It still works over my iPhone 4, but it's gone on my computer. Tried restarting iTunes but no luck. Any suggestions?