Office Chair return declined

I purchased a new office chair 2 months ago. When I came in to the office one morning, sat down and two of the chair's legs broke off. I have logged a return and after they assessed it they sent me an email saying the return has been declined as I did not look after the chair and it broke due to negligence. I have rep**** asking how do that they get to that conclusion as we have several chairs in the office and none of them broke. How did I not look after the chair. The chair is still under guarantee and therefore needs to be replaced. To date no reply yet and no contact number to call the directly