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My suggestion would be to do a hard reset. Hold the power & home button at the same time until you see the logo. Then release the buttons and wait for it to finish. This works on phones up to the 6s/6s Plus. If you have an iPhone 7 then hold the power and volume down button together until you see the logo. Then release the buttons and wait for it to finish. This should stop the process and allow you to retry it. Hopefully this will solve the problem for you. Thanks!