How do i know i need a whole new fuse box
You don't mention the year, make and model of the vehicle so, my answer will be as vague as your question.A gasoline engine needs four things to run: fuel, spark,compression and combustion. When you turn the key does the starter engage? If no, check for power at both sides of the fuse when turning the key.. If there is no power at the fuse when turning the key, go to the ignition switch. You have isolated your problem. If there is power at fuse then, go to the starter relay. If there is power going into and, going out of the relay then, check the starter. You have isolated your problem. If yes, check for fuel, spark (plugs and wires), compression(block, pistons, rods, Intake and gaskets) and combustion (small scale explosion from compressed fuel and spark, thus pushing piston). Your problem may be with a passive anti-theft system installed by the manufacturer. I wouldn't know because you didn't mention the year, make and model of the vehicle.Your problem could be with a collision based fuel shut down system installed by the manufacturer but, I wouldn't know because you didn't mention the year, make and model of the vehicle.Your problem could be with any number of common issues related to a specific vehicle; issues that may been discussed at length on this very site but, I wouldn't know because you didn't mention the year,make and model of the vehicle.