In SharePoint 2010, SharePoint Site Pages does not exist so when you migrate your content database to a newer version of SharePoint, it is advisable to move your default.aspx to the site pages.

To be able to do this, you will need to login to the site using SharePoint designer and go to “All files” then copy the aspx file.

Now go to “Site Pages” and paste the file. Once the file is copied on the folder, right click on the aspx file and select “Set as Home Page” to set the file as the home page of the site.

Once done, go to your SharePoint site using a web browser and you will notice that your URL has “/SitePages/default.aspx”. This indicates that the site is using the aspx file that was copied/moved to the site pages.