My phone will not turn on

If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support

Do the following:

Press and quickly release the volume up button

Then press and quickly release the volume down button

Then press and keep holding the power button (keep holding it past the swipe to power off) until you see the Apple Logo, then let go

Hopefully, your phone will return to normal operation after you sign back in.