Halo 3: ODST no longer free for me.

I bought Halo MCC when it first game out and so got Halo 3: ODST for free when it came out on Halo MCC. The thing is, now it tells me I have to buy it when I try to play it… But I already got it for free once and played it. Where did it go? How can I get it back? Did this happen to anyone else?