Try OnStar for any trouble codes, or ask an auto store to read the codes with an ODB2 device.
Software reset would be my guess. But if it happens again, there may be another issue.Somehow I posted this twice, ok three times if you include dutche’s heater thread too. Anywho, I pulled the fuse and examined it. Looked ok so I popped it back in. Like magic, the heat is working again. No clue why. But I won’t complain.