As part of its revised account service agreement, Microsoft now requires Xbox users to sign into their accounts every five years or risk potentially losing their Gamertag.
Only your gamertag will be affected, and not your purchases. If you fail to abide by the new rule, it might be made available for others to use.
Microsoft did not state whether or not the change will be applied retroactively, so you may want to log into your account in case you haven't done so since, say, late 2011.
Microsoft regularly releases lapsed or inactive Gamertags back into the public domain, but this is the first time that the company has instituted a time frame. The new policy also applies Skype usernames.
You can find the full list of changes (gaming and non-gaming-related) on this handy page
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