Hi Sidhin,
Could you please tell us where is the mailbox located, on-premise or online?
Based on my research, I have found there is a KB article about How to troubleshoot deleted user accounts in Office 365, you can refer to the Resolution 3: Recover a user account that was deleted because the on-premises user object was deleted from the on-premises Active Directory schema:
http://support.microsoft.com/kb/2619308/en-us
If you have tried this and can’t recover it, we have a group dedicated to dealing with the Exchange issues, I recommend you post the question to the Microsoft Office forum. This will help us solve the question more quickly.
http://social.technet.microsoft.com/Forums/exchange/en-US/home?category=exchangeserver%2Cexchange2010%2Cexchangeserverlegacy
Thanks for your understanding.
Best Regards,
Jason Jiang