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Try holding down the power button for 20 seconds, or until the iPhone shuts Off.
Leave it Off, to cool down and reset, for at least 20 minutes.
Then turn it On, and try again.
This sort of problem often enough happens owing to simple overheating, though even so it's not the best sign in the world.
Since I have not heard back from you, I am assuming that my advice was followed with a good result and my answer was helpful.
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