Sorry to hear about your Nest thermostat but thanks for reaching out to the Google Nest Community.
Being offline can be caused by several reasons such as the thermostat lost power or the battery drained or it lost internet connection from your router. Just to confirm, does this happen randomly or on specific times of the day or night? Also, are other devices on the same network still connected when you receive these notifications that your thermostat is offline?
While we highly recommend that someone to be at the thermostat's location to further do some troubleshooting steps, I would also want you to check on suggestions here.