iPhone StandBy/AOD not working after update

My phone updated overnight, it's now at 26.3.1 (a). I noticed when I woke up that standby mode was off and now I have no 'always on' display. My phone screen just goes dark--black--and the only way to see the display is to push the side button. In fact, the phone is completely unresponsive until I push the button. I've been through all my settings and it doesn't look like anything has changed, but I'm new to iPhone and I'm not sure I know everything to check.