Both NAO and Pepper can be programmed to respond to a variety of stimuli, or programmed to engage in back and forth conversations. This is not a default behavior however, and your NAO or Pepper will not respond to any conversation until you teach it how to.
One easy way to do this is to install something called “Basic Channels” from the Aldebaran Cloud store. Basic Channels is a packaged set of dialog programs that makes the robot conversational right away.
If your NAO or Pepper used to respond to you and recently stopped, it is possible that the volume of the robot has been turned down so low that its answers are inaudible. It is also possible that a microphone is malfunctioning in the head. If you believe this to be the case, Contact Support.