Modern detectors are a bit different from models of the past.  They have more sophisticated and potentially heat-sensitive components.  for this reason, they will monitor the internal operating temperature and if the components are too hot, the detector will sound the warning and turn off.  This will prevent the detector from sustaining any heat related damage.  Your detector will operate again normally once it has cooled sufficiently.  

We recommend that during summer/hot weather, you remove the detector from the window when not in use so that it's not sitting in the sun in a parked car for extended periods of time.  This will help to ensure that it's ready to go when you are.  Otherwise, just give it a few moments to cool down. Once the temperature has stabilized, it will turn back on normally.  It's important to note that this is normal behavior and does not indicate the need for service.