When the form stops syncing with Google Sheets, you either have:
changed the order of columns in the Google Sheet (please don’t change the order and add new columns to the end)
or you’ve changed the title of the sheet. in this case, you should reconnect the sheet to your form.
In both cases, you can re-sync data from the re-sync button.
Remember when you are adding your Google Sheet, the sheet title is at the bottom of the page, and you should use the related sheet URL.
For example, if you want to change sheet 2 to sheet 1, the title and URL of the sheet will be different and should be changed.