How do I recover lost achievements?

Solution 1:

Ok, if you are not connected to xbox live when you earn an achievement that achievement is stored on your local box.

If you then go and recover your gamer tag to a new box, the new box (and live.xbox.com) won't show those achievements. To have your achievements unified across all media you need to sign the box that earned them back into xbox live on that account.

Recovering your gamertag is a special processes where your gamertag is transfered to a new box and retroactively removed from all other boxes. Because you removed it from his old xbox before updating that achievements, you necessarily lost your ability to update those achievement.

I'm sorry, but you should always connect to xbox live whenever possible if you want to maintain your achievements. God forbid your box red rings and you lose all your work.

Solution 2:

From this support page:

"Any achievements or changes that you have made since the account last connected to Xbox LIVE will be lost during the recovery process."

It's sad, but no luck, it's gone for good.