If you have not updated your OS to the latest version from Apple, you may experience issues connecting and disconnecting with the Apple TV. Some staff are experiencing this:
They "disconnect" and the picture disappears from the projector, but the icon stays blue indicating the Macbook is still connected to the device - even though others can connect. Further, the Macbook cannot reconnect to the Apple TV when in this state.
If this happens to you, here's a work around. You can either restart your Macbook, or:
Click the Apple icon on top left.
Select System Preferences.
Select Internal Speakers as the audio output device.
This will complete the disconnect from the Apple TV and allow you to reconnect.
Best practice is to make sure your Macbook is up to date with the latest updates from Apple.