This problem is brand new this morning. When I boot up the game, the splash screen with the GT comes up and then it goes to the intro with the Turn 10 thing as the cars drive by. My game freezes every time right as the cars are driving by. I have unplugged and replugged my xbox, I have uninstalled and reinstalled, I have “quit” the game and try to restart it, nothing is working. I tried other disced based games as well as download only games and they all work fine, I am only experiencing this issue with Forza. I played it last night so this issue is new within the last 12 hours. Anyone have any suggestions or something else I could try? Has anyone else been experiencing this? I really like this game so it would be a shame to have to retire it forever.
Plugged and unplugged? Have you done a full, hard reset of the system:
Try doing a system reset (hard boot) as below:
1 – from the Dashboard, out of all games, hold the X button on the front of the console down until it shuts the system down.
2 – Disconnect the external power from the wall plug (or at least disconnect the wall-side connector on the power pack itself).
3 – WAIT until all power drains from the external power pack (no light, either white or dull orange showing on the pack), then remove the PSU-to-Console connection from the back of the Xbox One.
4 – After about three minutes, reconnect the plug to the console followed by the power supply to the electrical outlet.
5 – Restart the console in the usual manner, i.e., either with the controller or a short push on the X button on the front of the X1.
6 – WAIT until the console self-checks and fully boots to the Dashboard before attempting anything.
If you don’t disconnect the external power pack and let the power completely drain out, the system is still retaining “old” stuff and not resetting. If there are any updates (system or games) following the reset, let them complete 100% before attempting to play any game even if it says “Ready to start” in the downloading queue.
I did this and to no avail. I also really, really do not want to have to reset my console to factory settings. I just hope that Turn 10 will release an update to remedy this issue. Still open to any suggestions anyone has!
I am also having this problem, exactly the same as you. I have deleted my profile, uninstalled, reinstalled, hard reboot etc. and none of it has worked, i think it must be something to do with the update and T10 are gonna have to do another update to solve the issue… if they think im buying DLC after this… they can think again…
But just thought id say just so you know it isnt just you. Good luck 
EDIT: I have resolved the problem, just reset your console to factory settings, because all your data is saved to the cloud your data is retained. You may have to reinstall the game (unless you put all your games on an external hard drive) then you just start them, let your data sync and get going again.
I can confirm this - two of my friends have the exact same problem: None (!) of the Forza-Games are working. Forza 6 crashes back to the dashboard after the Turn 10 logo has been displayed.
Nothing helps - Hard reset had no effect, deleting and re-downloading the game, player profiles and savegames didn’t work either. Even a revert to factory settings didn’t help. Forza 6 simply doesn’t start, this goes for both the disc-based version and the digital download.
Needless to say, my friends are beyond frustrated. Microsoft Support was also unable to help. For the record: Their consoles are not connected in any way.
Something is wrong big time… Any help is greatly appreciated. Turn10, please look into this, it’s a serious issue! Thanks!