@Stephanie3
Did you change your Wi-Fi? If you did NOT, you could try just rebooting your doorbell by turning off the circuit breaker for the circuit that includes your doorbell for a minute or so to see if that brings it back online.
I don't know if this will be of any help, but this video at about 8:28 shows how the hex screw is tightened:
https://support.google.com/googlenest/answer/12570869
If your hex screw is stripped or broken, you could try contacting Support:
1. Go to https://support.google.com/googlenest/gethelp.
2. Select a category/product family, and type in a brief description of the issue, then click "Next".
3. Click "Other", then click "Next step".
4. Under "Resources", just click "Next step". (Clicking one of the listed links will bring up a Help page page.)
5. Under "Contact options", choose to get a phone call or chat with the team, depending on availability in your country.
