If you suddenly can no longer see your thermostat and your two cameras in either the Google Nest app or the Google Home app, one common reason is that you've inadvertently logged into either the Google Nest app or the Google Home app with the wrong Google Account.
Fred246, we saw your post and wanted to chime in.
You can try to uninstall and reinstall the Nest app, then choose to sign in with Google with the same credentials you originally used for your Nest devices. After you sign in, try to switch home structures if you have two or more homes to check other Nest devices.
Also, you can check this guide for additional troubleshooting steps.
Thank you for your help, MplsCustomer.