If your Gamertag is linked to an e-mail address with the possibility of expiring, might want to change that… quickly. If for any reason the e-mail address should expire (Hotmail for example), then you run the risk of losing your Gamertag.
There is currently no fix for the issue, short of creating an entirely new Gamertag minus achievements, friends lists, etc. It’s reported that only a small number of users ever experience the issue, but it’s also one many people probably aren’t aware either. So, consider this a public awareness post, unless of course you already knew this, and in that case, why are you still reading?