Title. All locked achievements, including time consuming ones like multiplayer wins or watcher kills have gone to 0%. What’s the deal?
Hmmm same has just happened to me 
guess we’ll just have to wait and see
same thing happened to me this morning, everything is back at 0% now
I also have this same issue. And my achievements for beating Requiem’s par score never popped nor did the achievement for simply completing the mission.
Mine have reset back to 0%. I’ve collected a few toys in the Halo 2A campaign, and beat the par score on 343 Guilty Spark, but nothing is unlocking.
It’s all due to the hack from last night. The achievements are stored on the servers that were hacked; the target was the xbox live profile server which is why everyone had such a hard time signing in. As far as I know there is still maintenance going on so you should have everything back as soon as everything is fixed. After that, all you have to do is turn your xbox off and back on, sign in and everything should be back! 
> 2533274953034359;6:
> It’s all due to the hack from last night. The achievements are stored on the servers that were hacked; the target was the xbox live profile server which is why everyone had such a hard time signing in. As far as I know there is still maintenance going on so you should have everything back as soon as everything is fixed. After that, all you have to do is turn your xbox off and back on, sign in and everything should be back!
I hope you are right. This just adds to an already disheartening situation.