Volume buttons stopped working after battery replacement.

So I thought I had done a successful battery replacement on GF's iPhone 4. It seemed to go fine and phone worked afterwards. She is getting long life from battery and she needed this as her phone wasn't making it through even half a day. However, the volume buttons on the side no longer respond.

I must have hit something inside when doing the replacement. Any idea what I did? And how to fix it?

Lack of buttons isn't end of the world as software volume control still works fine. But I'd like to fix this if I can. I wonder if I pulled off a ribbon accidentally.