Give the dead side a firm tap while playing some music and see if it comes back to life, even if for a few seconds. If it does, the speaker is most likely gone and you'll need to replace it. I have written a guide for [http://blog.oxplot.com/qc15-repair/|repairing the dead speaker of QC15] but it might be of help to you, at least to give you pointers.
The other possibility is that somehow one of the wires carrying signals to the affected side is disconnected. You'll need to open it up and see if playing with wires where they are soldered to the board brings the speaker back to life.