iPhone 12 will not restart. can see icons but they are unresponsive and pressing side button doesn't work

Force Restart:

Press and quickly release volume up button

Then press and quickly release volume down button

Then Press and KEEP HOLDING the side power button (keep holding past the swipe to power and past when the screen goes black) until the Apple Logo appears, then let go.

When you sign back into your phone, see if normal operation resume.

You're welcome. Remember this procedure. Use it after every iOS update and it usually cures most little things which can go wrong on your phone.