I have built my website with cornestone. Yesterday, I’ve changed the code with “text” option to see what will happen on the page. I’ve uppdated the site. Today I want to go back on the cornestone editer but when I click on it, it’s like a new page : all the sections that I’ve built have been crushed
Once you have edited your page outside Cornerstone you broke it´s code. You could try to edit the changes back to the original and see if it will work, but we cannot guarantee.
If your hosting provider has a backup, and most of them do, you should restore your website to the version previous your manual changes.