Of course you want to ensure that the problem isn't with the remote (make sure that it works with a Mac, for example). If it tests out okay, then turn to the Apple TV. Disconnect its cables and take it to your Mac.
You'll need a USB cable with a Micro-B connector on one end. (Many digital cameras ship with such cables.) Connect the tiny end of the cable to the equally tiny USB port below the HDMI port on the back of the Apple TV and plug the other end of the cable into your Mac. Plug in the Apple TV's power cable and launch iTunes. The Apple TV will appear under the Devices heading in iTunes' Source list. Select it and click on the Restore button to the right. Confirm that you want to do this by clicking the Restore and Update button that appears. Agree to the license agreement and iTunes will begin to download the Apple TV software (it's around 540MB). When it finishes downloading the software it will install it on your Apple TV.