Your Nest may be stuck in "Safety Temperature" mode. It's happened to us and to a lot of other people on here. If your house temperature falls below the Nest's Safety Temperature, the Nest calls for heat even if it's off. Once the power is restored, you SHOULD be able to take control again and change the temperature manually or according to your schedule, but there's apparently a bug affecting some models. The workaround is to go to Settings and change your Safety Temperature to be well above or well below the current house temperature. That may get the Nest back to obeying you again. Another option that may work is to do a Restart. If you search the forum for "Safety Temperature" you'll see a lot of discussion.
