My iPhone 11 is frozen and will not reset by using the volume up down and power key. What should I do?

You need to follow this precisely:

• Press and quickly release VOLUME UP

     then

• Press and quickly release VOLUME DOWN

     then

• Hold the SIDE button until an Apple logo appears (15-30 seconds), ignoring "slide to power off" if it appears

The operative word here is QUICKLY; and try it a couple of times, the timing is a little tricky.

If this doesn’t work after a few tries your phone has a hardware problem. Contact Apple support to discuss repair options. You can click on the Support link at the top or bottom of this page.

Continue following If your iPhone won't turn on or is frozen - Apple Support