If your situation is similar to mine, I had a power cut (power outage) that happened when the outdoor temperature was below freezing. As a result my Nest Hello (Gen 1) wired doorbell never rebooted when power was restored. Solution: Disconnect it, bring it inside, power it up on USB (leave it alone for about 30 minutes to heatback up to room-temperature). Then reinstall it (quickly) while back outside. It's now happily wired-back and online.