Have you changed your gamertag recently or sometime after Halo 2’s servers went down? If you did that explains everything. For stats to follow your account. And you need to play the game as your new gamertag so the stats will return.
So for Halo 3, Halo 3:ODST, and Halo Reach. Just play a game on then for the stats to return
For Halo 2 I am sorry to announce your stats will not return to you.
If you didnt change your gamertag between the closure of Halo 2’s servers and 03/31/2012 then Bungie.Net will have the stats there as they are archived there. http://halo.bungie.net/default.aspx(use the search-bar for your old gamertag, and also select Halo 2, if nothing pops up then your stats arnt saved)