You might have noticed that Forza Horizon 3 won't start up when it tries to launch. Although you might think it's an issue with your install, it's actually widespread and not related to your computer.
This is caused by problems with the authentication server. Upon boot, the game sends packets to the server to verify that it's a legitimate copy before establishing a connection. If this fails, then the game will not proceed to the main menu.
Unfortunately, you will need to wait for the problem to be resolved by Microsoft.

Embark on an inspiring mission of human connection beyond the UCA. Sam — with companions by his side — sets out on a new journey to save humanity from extinction. Join them as they traverse a world beset by otherworldly enemies, obstacles and a haunting question: should we have connected?