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.