I had until recently owned Halo Reach. It suddenly disappeared from my account and it says I don’t own it anymore. I had it on the 360 and want to play it on my Xbox one. It was a digital download.
I would double check that you’re checking the correct account first of all (as silly as that may sound), would also make sure you didn’t have it shared via another Xbox Live profile. If you’re confident you purchased it on your account it should be listed in your purchase history - I would reach out to Xbox Live Support for further help with that