i signed into my xbox live account on my computer and it changed my gamertag to HeardWord5503 and it has none of my achievements or anything
Sorry to hear about your situation.
But, you will need to contact Xbox Support (http://support.xbox.com/en-US/). They are the only ones who have access to the accounts.
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Yes, contact support by choosing option 2 when you dial your national support number (see first link below)