sound output only lists internal speakers when usb is connected to dac

This problem happened after I had been using my mbp for months as the music server in a stereo system. When I have the mbp connected via USB to an external digital to analog converter (dac), the audio preferences had always listed the dac. Now it only lists internal speakers. I've tried rebooting, checked my UBS cable, checked my dac. The only thing I've done recently is create a playlist and sync a new iPod to copy the playlist there. I'm not sure if that could have done anything to create the problem of missing dac, but I am a relative novice on computers.

Anyone have this problem or know how to fix it?