The only people who can help you are GMail:
Here is the link to gmail support website : http://mail.google.com/support/bin/answer.py?hl=en&answer=46346
On the other hand, if you have not used your Gmail account for over 30 days, it may have been deleted. You must then create a new one, insuring that you note your password and security question and store it a safe place, remembering where the safe place is.
P.S. Since it was so important, how did you manage to forget ?