Sterns poor service and breaking a guarantee

I bought a gold chain in 2011. I received a lifetime guarantee with it. They told me the chain would never break. I took it in a few times for repairs. The chain broke in half in the height of Covid 2020 However 10 days ago, when I took it in I received the worst possible service. The salespeople already told me it broke because of negligence. I phoned them now to hear if it will be repaired. They told me I have to pay. What does a lifetime guarantee mean. I did not break the chain as I was carrying an expensive opal on it. I do not have insurance.