Sounds to me like you had a sudden ignition failure that led to the backfire, not that the backfire caused this problem. Like Mike says, you need to start from scratch on your ignition system, check for power, and check all of your fuses - a cheap probe type test light can check fuse status pretty fast using the little test port holes atop each fuse, power to the fuse, and power through the fuse with two touches. And welcome to CHR! Tell us more about this 5.7L if more questions - what's it in, what year, carb or injected, ignition type, etc.