I can't start my 1997 Mercedes-Benz C200 after replacing the key fob battery.

Unfortunately, the only simple solution to your problem is find the little circuitboard (chip) that went missing when you did the battery replacement. Without that, the key can’t talk to the car’s computer and let it now that it’s okay to allow the engine to start.

The alternative is to find a car locksmith and have a new key programmed to talk to the car. Exactly what else this will involve will depend on the car itself and how the security system is configured. You may need to change other components on the car and it could get pretty expensive. You may also need to trailer the car to the technician.

If that doesn’t pan out, perhaps try a Mercedes-Benz dealership which should be able to sort it out and get the car running. But you’ll be paying full price for that service.