I'm using 11 Pro and just updated to 17.4.1 2 days ago. Everything was good.
A few hours ago, the phone started playing up. No app could be launched, not even the System app. Pressing the side button won't switch the screen off, and I had to wait for screen timeout. When the phone is off, I could press the side button to turn it on.
I tried the restart by pressing the side button and either of the volume button (Restart your iPhone - Apple Support (NZ)) but even that screen (shown here) was frozen.
Thought I'll wait for the battery to drain. Until I found out that there's another restart method of pressing the up, then down then side buttons individually (Force restart iPhone - Apple Support (NZ)) which worked!