we repaired my frontier head gasket and put everything back together, now we have no fire and gas. how do we find out why?

Hello, no one could possibly answer this question without a hands on evaluation of your vehicle, i'm sorry. To have torn the engine down and reassembled -- contacting many different components and harnesses - it's impossible to know what has happened. The first place to check is ensuring all fuses are intact, in case something was blown during your repairs.