For a car key remote to work securely, it has to be effectively paired with the receiver unit in your car. Then someone with the same make and model can't walk up and use their fob to unlock your vehicle.
If your keyless entry remote and your car are no longer on speaking terms, you will have to reprogram your car's keyless entry system to regain your car key remote functionality. You can accomplish this by turning the key in the ignition several times with the doors shut.