What causes the computer in 2013 Cadillac xts to mal function an d not start car

Hello, the "computer" is extremely unlikely to be malfunctioning. What is more likely is that some kind of input to the engine's processor is failing, resulting in your vehicle not starting and having no electrical functionality. Even more likely is a faulty engine battery -- or one that is low in charge -- preventing electrical functions from working and keeping the car from starting. Either way - it's too hard to know how much repairs will cost -- troubleshooting is needed first.