I had a calendar event pop up on my iPhone that was giving me 2 week advance warning of an upcoming event. I kept hitting snooze since I wanted it to remind me again later.
Somewhere along the line, a crescent moon icon showed up at the top of my iPhone screen. I assumed that it meant that I had a pending alert for my snoozed calendar events.
Come to discover today, that moon means that I had managed to somehow fumble the phone into "do not disturb" mode via the swipe-up configuration menu. This would explain why I've been missing calls and texts a lot over the past couple of days.