Settings
If the Plantronics headset was working before, an unintentional change in settings may have resulted in the headset not pairing. Access the Audio Device Settings menu in Settings, and restore the default settings for headsets.
Re-Pair
Resetting the connection between the headset and PS3 may solve the pairing issue. USB headsets only need to be connected to a USB port to be paired. A Bluetooth headset must go through the pairing process (see Resources).
Functionality
If you're using a Bluetooth headset, confirm that it has fresh batteries and is receiving power. If possible, sync the headset with a cell phone or another PS3 to confirm that it works.