Cannot connect MacBook Pro to AppleTV

I went to try and stream a video from my MacBook Pro today and the mirroring icon did not appear at the top of the screen. I went into settings, Displays and AirPlay Display, clicked on AppleTV. I get a message "Cannot connect to Apple TV". It doesn't tell me why. I can stream from my iPad and iPhone so the AppleTV box is not the problem. I need to stream from the computer and I cannot figure out why this won't work. It used to. I tried resetting the AppleTV box but that did not help.