Grayed Out Songs on iPhone

I just got an original iPhone replaced by Apple under warranty. I've put all my backed up music back on the iPhone. However, when I'm in iTunes on my computer, I can't seem to get into the music on the iPhone anymore. All the songs are grayed out. For example, if I want to move a song into a playlist, I can't, because with the grayed out songs, I can't click on anything.

Any idea what I did wrong?

The iPhone itself works fine - I can play all my music that's on it. But it doesn't seem to want to interface with my computer.