Hi @CarolineP , sorry for the late reply.
It is great to hear you already checked the power at the location to confirm it is ok. If you still need assistance there are a few more details I would need:
1. When did you first notice this behavior?
2. Was it installed by a Nest Pro or by a heating/electrical engineer?
3. Did you notice any message on the thermostat display?
4. Did you have any maintenance done on the heating or electrical system recently?
5. Do you have only one or multiple Google Nest thermostat devices at the location?