If your iPhone 6s is relatively new and simply refused to switch on one day, you can check its serial number on the program's official page and see if it's eligible for free repair. Take note that the company says it "may restrict or limit repair to the original country or region of purchase," so you may be out of luck if you bought your device overseas. If your phone is eligible for free repair, though, and you already paid to have it fixed, you can contact Apple for a refund. The program will cover all affected phones for two years after purchase, so the program will likely be available until 2021.