If you switched the ignition back on whilst the airbag harness was disconnected (for instance to test your Vidcam), then the ECU would have stored an airbag fault. This will need clearing either by the dealer (however they may charge you as it's not warranty), or find someone local to you with VCDS who can clear the fault code from the ECU.
Until this is done, the car will still assume an airbag fault and disable that unit and warn you on screen.