Car keys

I brought in my car for service at Lindsay Saker vw Eastrand , later on I went to fetch my car only to find out that my key is no longer working when I press it nothing happens it can only operate manually, Christine the service adviser says that is the bettery. Now I changed the bettery with a new one still nothing, took the car to them and I'm being told that I have to pay, the key is broken and is my fault while it was working when I brought in the car. PLEASE HELP