I assume when you say the car won't start after getting gas, that it cranks over but won't start. If that's the case, there can be a few things that could cause that. Like Bill F said, overfilling can do it. It could also be the evaporative emissions purge valve. You could also have some sediment in the fuel tank that gets stirred up when you add fuel and that blocks the filter. Once that settles it can start. This can definitely happen if you've had to get gas at a sketchy gas station. Places with old rusty tanks that don't change their filters can cause all kinds of problems. It could also be a fuel pump going bad. The pump is inside your fuel tank and gets warm when it's running. Adding cool fuel can make a warm, almost worn out pump motor bind up until the whole thing cools off and then it can start working again. Eventually it will quit working altogether.