This happened to me as well. iPhone Xs (Model no.: MT9H2QL/A), iOS version 14.0.
Suddenly the device went to black screen with the spinning boot thing on the screen nothing else. It was stuck in that loop all day, until I could get back home and connect it to my Mac and reset the device.
I see from the forums that this issue was present in the iOS 14 beta. But looks like they have not been able to fix it in the Public 14.0 version either.
Kathors
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