I have a mazda 6 '08. It was low on fuel. I left for vacation, came back and now it won't start. All the lights came on, so I know it is not the battery. I put fuel in it and still the same.

Batteries can be deceptive. It's best to remove and fully charge the battery, which most auto parts stores will do for free. Lights require little battery amperage, starter motors require a lot.

Also, ensure you have more than just a token amount of gas in the tank. Turn the ignition switch to ON, until the fuel pump stops running. Then, turn the ignition switch OFF. Repeat this process several more times. This will fully pressurize the fuel rail. On the final attempt, turn the ignition switch to START. It should fire right up.