Hi Michel,
If the 404 error message occur when you click on the SharePoint icon, this might be caused the SharePoint root site is deleted or the default home page for the root site is deleted.
May I confirm if you are the SharePoint administrator in your tenant? If so, please navigate to the SharePoint Admin Center > Active Site, check if the root site is listed.
Or you could try to access the default site pages URL for the root site:
https://contoso.sharepoint.com/SitePages
See if the library could be opened without any issue:
1.If the library could be opened, check if there is no pages in this site page library. You could either create a new page and set it as the home page or go to site recycle bin to retrieve the previous home page:
https://contoso.sharepoint.com/_layouts/15/AdminRecycleBin.aspx
2.If the library URL also return 404 error, this means the site might be deleted. Please inform your admin to retrieve the root site for SharePoint.
Please keep us informed if there is any update.
Regards,
Alex Chen