It sounds like your pairing has been broken in some way.
Try restarting the computer, the Apple TV and your router to make sure it's not some sort of temporary network problem, if the problem persists:
Navigate to Settings > Computers > Your Library on the Apple TV, you may be told you will lose all synced content, but you can sync it back afterwards, you won't lose any purchased content that has yet to be transferred. Then select Settings > Computers > Connect To iTunes, note the passcode that appears on screen, click on the device in iTunes and enter the passcode when prompted.