Previous Account

Lending your account is against the terms & conditions,also,the chances of your account actually being hacked are extremely small.

When an account is banned you forfeit any content bought with that account.You cannot transfer purchases between accounts.

If you know the Gamertag for the old account you can try and contact xbox support by going to support.xbox.com/contact-us and talking with a support agent and if they can verify ownership of the account they can tell you what the email address was that
was associated with that gamer tag.

Sign into your Xbox again (using the new account), then change email address to something else. Then you should be able to sign into your original account with the original email address.

Change the WLID associated with my gamertag.

If for any reason you want to change the WLID you have associated with your account, all you need to do is follow these simple steps.

1. On the console, go to My Xbox, and select your avatar.
2. In Account Management, select “Manage Account”.
3. In Your Information, select “Windows Live ID”.
4. Select “Change Windows Live ID”.
   1. If you have another Windows Live ID, select “Yes”.
   2. If you do not have another Windows Live ID, on a computer, go to Welcome to Windows Live ID, and click “Sign up”.
      
5. Enter the password for the Windows Live ID that is currently associated with your gamertag, and then select “Sign in”.
   
6. Enter the email address and the password for the new Windows Live ID that you want to associate with your gamertag, and then select “Sign in”.
   
7. Select “Yes, change” to change the Windows Live ID associated with your Xbox LIVE account.
   
8. Select “Update Contact Information” to update your contact information, or select “Done” to finish.
   

Additional Information

Your Microsoft Points balance will automatically be transferred to the new WLID.

http://forums.xbox.com/xbox_forums/xbox_support/f/12/t/19163.aspx

Please refer to Forgot the Microsoft Account Email Address for your Xbox Live account? Learn How to Find It.

If you are unable to find your Microsoft Account through that method, you might
try to contact the official support for help.

However be aware of the following: In the past, malicious people (ab-)used support agents to gain access to other people's account by providing personal information that could easily be pieced together by stalking people on social media (e.g. looking when
other people congratulated them to their x-th birthday and getting the birthdate, or checking
GPS Exif info from pictures of their pets to get an approximate understanding of their home address etc.).

So Xbox Support usually assumes that you are an unknown person trying to gain access to someone else's account. You will have to provide A LOT of personal information to really prove you are the owner of the account. Ideally you know the bank account/credit
card information used to buy some stuff etc.

If you can't provide that information (because you forgot that as well), then I'm afraid, even support can't help you, and you can't get your account back 😞

@Daft Badger I think he meant that his friend was hacking on a game, leading to his account being terminated. And @kbr369 Daft is completely right... the only thing you can really transfer is offline DLCs, since it is saved on the console, anything attached
to the account is lost.