Can you still get into your Gmail account? If you can the following may be useful to you:-
jay0lee got your back
If you back up from your old account following these instructions, and then restore to your new account, then all the emails should be transferred to your new account, including the folders etc that you may have had. It has an added advantage, and that is that there will be a back up of your emails on your hard drive, and you can back up with this on a regular basis.
If you can't get into your old Gmail account I would suggest that you try resetting your password by using your old username and the "Need help?" link on the login page.
If all else fails I fear you will need to contact Gmail, explain the situation and get them to re-instate the account.
In many respects if you rely on Gmail, the programme download mentioned above could be useful in maintaining a back up of your Gmail on a PC.
Hope this is helpful, if so would you please register that with Fixya