I had this problem and the first thing I did was turn off the console and unplug it. After plugging your console back in, go into your XB1 settings then select "Power & Startup" then turn off the quickly resume game feature. If you want to leave this feature on there are two things that you can do to hopefully prevent this from happening. Once your XB1 turns on and logs you in, wait a few seconds before you try to access the game. The second thing you can do is setup your XB1 to instantly log you in the moment your console is turned on. I can't remember exactly where in the settings you'll find this but even with this feature turned on, wait a few seconds before you try to start your game.
My apologies, I didn't realize that you were talking about Forza Horizon. In that case, just turn off your console and turn it back on. That should solve the problem.