Check all the connections into the rx, unplug one servo at a time and see if it comes back to life as a dead servo can some times short the system out, RTR radio systems are usually problematic at best and there are some good cheap upgrades available. Also check by wiggleing the battery holder and batterys for the rx and make sure the connections there are good, those little plastic 4aa holders can be fragile .

Also look into a failsafe, goes inline with the throttle servo wiring and makes it so if it loses signal it will set the servo where you set it to, most likely full brakes. but if its losing power and not signal wont really help.