Double check the battery. When you try to start the car, have someone monitor the battery voltage. It should be over 12 vdc and stay over 12 vdc when you turn the key. If it drops to near zero, your battery is shot, try a new battery. A long time ago, I ran into a battery with high internal resistance, it would show normal voltage until you tried to start the car, then nothing. When you say the car has no power, do you mean that nothing else works? Try the same test I mentioned above, monitor the battery voltage, and try pressing the brake pedal or turning on the headlights. The voltage should stay above 12vdc.