Likely an issue with "grouping" or not your musical sources.
You may need to delete and reinstall the app. Should then walk you through the process of getting it all running again.
Else, you need to look at the bottom of the screen, and see what music source it is pointed at. You can touch that to get it to open a list of available sources.
I use it to control both my main player, and my iTouch (where I bought the in app FLAC streaming so I can listen to my entire collection on my portable device when in my wifi environment) that I listen to with headphones.