Are the tracks in the session playing back for you? If so, you can ignore the second paragraph. If they are playing back, are the individual tracks on Waveform in the edit window? It might be possible that they are on some other automation parameter. If that's the case that will prevent editing.
I would think either way it might be worth resetting your I/O to default anyway - sometimes that jump starts things as well. I would also double check that your interface is listed as the selected device under 'Setup / Playback Engine' as well.