They'd been going through a difficult divorce and had some sudden caretaker responsibilities for an elderly parent land on them at the same time.

They went AWOL for about a week. Just stopped coming into work. The team, the boss' supervisor, etc. heard nothing. Eventually the company was able to make contact and all we heard was that they'd be taking an extended leave. They came back to work several weeks later, ostensibly as good as new, though I'm sure the real issues are ongoing.