I subscribe to the weekly newsletter on several sites, including here on Meta.SE. My Meta newsletter always comes on the same day as two others, so when I got the other two today but not this one, I went digging. Apparently I didn't get it last week either, but didn't notice then. It's not in my spam trap.
I then went to the subscriptions page on my network profile and found that my Meta subscription was missing. I've now resubscribed, but how did I become unsubscribed in the first place?