Hold Volume Up, Volume Down, then the Side button until the Apple logo appears to force restart your frozen iPhone.
yeah bryleeeaton is totally right here. if your screen is actually frozen and you can't even touch the slider to power it off (like most people are suggesting), then you gotta do the quick press volume up, quick press volume down, then hold the side button until the apple logo pops up. that's the real 'force restart' for when it's totally unresponsive. the other methods only work if you can still drag the slider tbh.