Hello. My name is Michael. I can help you with your question.
If you are absolutely certain that the new battery is good (if it was hanging around their shop for a long time, it might not be), I would suggest cleaning the battery contacts. To do that, I would recommend a chemical specifically used for that purpose. There is one called DeOxit that will clean the contacts, and not damage the camera. Here is a link to that.
If this doesn't do the trick, then it would have to be something internal... but this problem is the most common, so I would definitely try it first.