BarbErik:

I know exactly what happened... it happened to me also. Ok so here goes:

 

When you were sleeping master chief snuck into the room where you keep your xbox and infiltrated it. Then proceeded to place the cortana chip into your xbox with a standard UNSC issue USB adapter. As he was exiting he got into a fight with marcus fenix who was also coming into the room to get info on the locust horde. Ya see, your mother is one of the Locust Queen's poker buddies. While master chief and marcus fenix were fighting the locust came up from underground and broke your T.V. So they went to walmart and got you a new one. Then when they were away Marcus attempted to chainsaw master chief with his lancer, and a fragment of MJOLNIR armour flaked off and hit your xbox. Since Halo Wars was in your disc tray. You were blocked from getting Achievements for that game. Now what happened was marcus fenix was killed and had to be dragged to the bay. Master Chief came back to clean up the mess and found the locust's there installing your T.V. and had a tea party. Then when dumbledore and his flamer buddies found out there was a tea-party, they came and broke it up because they weren't invited. Because Dumbledore was a nice guy, he didn't want to damage your house and subsequently teleported the brawl out of the house. 

 

So basically your achievements are blocked by a scrap of MJOLNIR armor.

sounds like the problem, ehh