Walmart

I purchased a Microwave from my local Walmart and about 3 weeks later I was heating my supper and it completely shut down. I was afraid to use it after that because one side of it was extremely hot so today I took it back to Walmart to get either a replacement or a refund. I had the receipt but no box (because who keeps a box when you plan on keeping the appliance?) They would not take it back even though I told her it didn't work. I don't understand why they couldn't take it back when I had the receipt and the papers that came with it. If I was supposed to keep the box they should have told me when I checked out with it. I'm really disgusted with their so called customer service!