If data has stopped syncing between your app and its Google Sheets data source, follow these steps to reset the connection:
Verify that the email address used with your Glide has read/write permissions to the Google Sheet used by your app.
Ensure you are signed into your Google account and make sure you are not signing in via Single Sign-On (SSO).
Sign out of Glide and sign back in using "Sign in with Google."
Check to see if the connection issue has been resolved by navigating to the Data View and clicking the manual sync arrow ↻ in the bottom left corner. If it still doesn't work, continue to the next step.
Navigate to Settings and then Data.
Click the 3-dot menu to the right of the Google Sheet name under the heading DATA SOURCES, and click Replace.
In the modal that opens up, find and select the Google Sheet used by your Glide app. Make sure you are picking the same file or a copy of it that contains the most recent data.
Click Select.
Navigate back to the Data view in the Builder and click the manual sync arrow ↻ in the bottom left corner.
Your Glide App should now by reconnected to its Google Sheet.