Check for loose cables. Recycle system power. Turn it off, wait 45 seconds, turn it back on and see if that fixes the problem. Make sure the codec is turned on with the rest of the equipment and that the main power switches are on. Check switch settings on peripheral equipment.
Video Make sure the local and far site video monitors are set correctly.
Check to see that the cameras can be switched from the control device. Check camera iris and focus. Make sure the camera is on the particpants in the room. Push the test button on the back of the codec to make sure the cameras still move. They may also go through a test sequence when you recycle power on the codec.