If Notion says that your Nest Thermostat is disconnected, the Notion app is not communicating with your Nest. Check your Nest app to see the status of your thermostat there.
If Nest says that your thermostat is connected and the Notion app still says it's disconnected, try a few troubleshooting steps in the Notion app:
- Hard close the Notion app and re-open it
- Log out of the Notion app and log back in
- Delete your Nest from Notion and reset the integration
If Nest also says that your thermostat is disconnected, then follow the link here to see some suggestions from Nest.