iPhone stuck on Lock Screen won’t respond to try to force restart
phone stuck on Lock Screen and won’t respond to hard reset
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Just to make sure everyone is on the same page, this is the procedure used to Force Restart.
If your iPhone won't turn on or is frozen - Apple Support
Also if iPhone 8 or later, the last step of holding the side button until Apple logo comes up takes longer than what you would expect. It can take up to 20 seconds.