New Dell XPS13 - stuck on Dell logo

Hi,

I just got my Dell XPS 13 back from factory repair/service with all functions tested. I was running Windows 10 before, but the Dell repair service re-installed Windows 8.1 during the repair. So I installed Windows 10 (the free version from Microsoft for users of assistive technology) and everything went smoothly. But once I rebooted, the computer is stuck on the Dell logo and will not load the OS. I pressed F12 and ran the Diagnostics, and no error was found. 

What can I do now ?

Thanks !!