To see the preview thumbnail you need to open and save the project. Then when you reopen QGIS you should see the image
Working from Linux here but the program is identical. The option you're looking for is under the general options -> "Project files" tab.
The option for opening projects on launch can be set to the "Welcome Page" (What you're looking for), to open up the last/specific project, or to simply show a blank canvas. It's possible that when you upgraded your installation, this setting may have changed.