iPhone, 12 will not turn on. I held the side button and press the up and down buttons. The Apple logo appears and then it’s dead.

• 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 does not restart the phone they you will have to restore it to factory settings→Restore your iOS device to factory settings - Apple Support