Make sure the headset power switch is in the ON position.
Your headset may be out of Bluetooth range or experiencing interference. Move the headset closer to your phone, and make sure your headset and phone are on the same side of your body.
Your headset battery may be drained. Recharge the battery using the supplied A/C power adapter.
The listening volume may be too low. Tap the volume button to increase the headset volume.