All my animation cels have disappeared and now there's just a "video group".

I've been making a cel animation in Photoshop (it's tedious, but I've done it many time before). I had over a minute of animation creating new cels at 10 frames a second. I had to put the project up for a week, and when I opened it again today, the cels are gone from the timeline, and now there's just a Video Group. I don't see a way to open it up to reveal the cels so I can work on them. The animation plays as far as I worked on it, but now I'm dead in the water. How do I open up this "Video Group" so I can add and/or adjust my cels?