Why won't my iPhone turn off with the side buttons?

Please try a force restart.

• Press and quickly release the volume up button.
• Press and quickly release the volume down button.
• Press and hold the side button until you see the Apple logo 

Learn to: Force restart iPhone - Apple Support

“iPhone won't turn off: my iPhone won’t turn off.”

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So, what’s on your screen?

I. Troubleshooting a Frozen iPhone:

Refer the the following for troubleshooting techniques...

• If the Screen isn’t Working on your iPhone or iPad - Apple Support

• If your iPhone is Stuck on the Apple Logo - Apple Support

• If your iPhone Won't Turn On or is Frozen - Apple Support

• If you Need More Help - iPhone - Apple Support

II. Fixing a Boot Loop:

Your iPhone can at time be stuck in a Boot Loop. This typically occurs when you go abuut installing an update, and it stalled while in the process. Go here to fix that: How to Fix an iPhone in a Boot Loop - User Tip