Key stopped working exactly 30 days after I made it

Keys run in 30-day windows. On a live subscription the window rolls automatically — so the usual cause of this is that the subscription lapsed rather than the key doing anything.

Check billing first. If the subscription is active and the key still says expired, that's a bug and worth reporting.

Card had expired and I missed the dunning emails. Renewed and the key came straight back. Entirely my fault.