iPhone goes black with spinning gray circle

I have a brand new iPhone 8. Was working fine for a couple of months; then yesterday it suddenly went all black with a light gray circle spinning endlessly in the middle. It didn't respond to any button presses (including holding power and main button, to try to reboot), although oddly, when someone called me, I was able to take the call over a bluetooh handsfree thing in my car, so some part of it was still running. When I plugged it into my Mac Pro, iPhoto started up (apparently knew it was there) but iTunes could not see it. I left it alone to let it run out of power, and when it rebooted, it was fine, but this morning it suddenly did it again - this is terrible because now it's at 100% power and it will take all day for me to be able to reset it. Anyone know what this is, how to reboot it when it happens, or how to prevent it from happening again?