iphone se hard restart

For the first generation of iPhone SE see this instructions

Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the side button. When the Apple logo appears, release the button.

for the second and third generation of iPhone SE, see this instructions

1. Press and quickly release the volume up button.
2. Press and quickly release the volume down button.
3. Press and hold the side button.
4. When the Apple logo appears, release the side button.

Force restart: Force restart iPhone - Apple Support