2005 Mercedes Benz C230 Alarm Key Not Working

First make sure the battery is properly seated. Next make sure that the contacts on the key are free of corrosion. If the contacts on the key look worn use a small piece of very fine grit sand paper to scuff the contacts on the key. Also if you have a second key test the other key on the alarm. If the alarm works with this key you know your problem is something with the key and not the car. There is no renormalizing process for just changing the battery on the key so there must be something simple that has caused this to happen.