"iPhone is locked. 'Diagnostics are in Flight': This alert just showed up: 'Diagnostics are in flight. Perform the following Send Feedback. Ignore'. But my phone is totally locked -- I cannot turn it off or perform any functions. It's like Apple has taken over control of my phone. I do not like this."
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Thank you for the screenshot.
Successfully Turning off on an iPhone:
A. Close All Apps:
These Apps may be open. So, save what need be, and then close all apps. Hold you finger down as you swipe up form the bottom-left corner of your screen. Then, close all of these open apps, by sweeping up quickly for all apps.
B. Perform a Force Restart:
Try Force Restarting your iPhone. Save any information that need be. Then perform the restart. See if you can then perform this. Go Here: Force Restart iPhone - Apple Support