mercedes 1999 c280 key not turning

What is your question?

I have mercedes 1999 c280，key is not turning，car not start. I bring to the dealer，they said their computer plug in my car can not scan because the car have no power. so their diagnose can not find anything. and they charged me double price for diagnose but not find anything. I don't know what to do. How can I fix my car？