I started up Forza Horizon 4, and as I was waiting for it to finish syncing I clicked cancel and lost all my save data to my game. I had around 200+ levels and over 600 cars (Most of them I had multiple of). I’ve tried searching around on the fix to this problem and could find nothing.
Send in a support ticket. I don’t think they can restore your save unfortunately, but I think they give you credits so you can buy back cars. Are you on Xbox or PC? If you’re on PC you can back up local saves in case this happens again.