Will a new sheet contain previous submissions?

for an unknown reason, my spreadsheet only contains 21 entries now, instead of 340.

All the others have disappeared.

So my question is: when I unlink the Google spreadsheet, and re-link a new one, will all the former entries re-integrate in the new one? Or will only new entries appear?

If you can see only 21 submissions out of 340, your integration is broken. Please note that editing of the Spreadsheet that is integrated with a form can break the integration, and it is not recommended. 

To resolve this issue, I would suggest you to re-integrate your form with the Google Spreadsheet. 

When the form is re-integrated with the Google Spreadsheet, all form submissions will be shown inside of your new Sheet.