iPhone 7S
iPhone 7S went black and won't power on.. Just a black screen.
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Do you mean 7 or 7plus? there is no 7S
Probably a hardware failure but see the process below, using the Update option if possible.
Work all through it.
If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support
There is no guarantee you can save any data since your last backup if you need to use the Restore option.