Where did the negative review come from?  If it is on your custom review page you can try and work it out with the reviewer.  If it is posted on an online review site you will need to respond to them.  Create your response in a neutral, non-emotional manner.  If you need help with this,  create a support ticket and let us know where the negative review has been posted and what it says.  Send us a response to the client and we will look at it for you and wordsmith it.  We will then get back to you with edited verbiage to response.  You then can reply to the client.