If you press one of the inputs, the remote then switches to take control of (what it thinks) is the source device for that input. If you press the amp button again, the remote then returns to controlling the amp.
Sometimes happened to mine when the batteries got low. I was advised by my dealer to remove the batteries last thing at night, leave it overnight and then replace the batteries with fresh ones in the morning. On the odd occasions that the remote froze, this method always worked.
Having said that, I replace the batteries regularly now and haven't had the freezing issue since.