Another LogicPro question: Why can I only hear my guitar track in Sample Editor?

I’m using LogicPro 8 on a MacBook Pro. I’ve recorded a song with lots of audio and MIDI instruments. Everything seems fine except that one of the guitar tracks (which was working before) has suddenly stopped playing back. I can still see the audio regions, and I can hear it (though without the effects) by going to the Sample Editor.

This happened right after I clicked on the “GuitarAmpPro” effect for that track. I tried to change the reverb, but then I saw it was “disabled.” I didn’t expect this to have any effect.

The regions also look different now. The top of them is light grey instead of blue, and they have an “O” icon before the file name instead of after it. For instance, the audio files in the guitar region are like this:

O Audio 10

The other tracks are all like this:

Audio 6 O

None of the tracks are muted or on “solo,” and the guitar track doesn’t have the volume turned down. Closing and reopening the application doesn’t solve the problem.

Any ideas?