Replace-by-fee Problem
Please i have an Account with BlockChain but i notice my account has a withdrawal of all my BTC which was not done by me. It has Replace-By-Fee
  What might have happened? 


If you did not initiate this transaction, then it looks as if your account was somehow compromised. Have you ever shared login details for the wallet with anyone else, or did someone create the wallet for you? There are common scams which employ these tactics. 

Otherwise, without more information, it isn't really possible to discern how your account was compromised. In any case, that wallet (and possibly compute you used to access it) should be considered compromised, and should not be used for any further BTC transactions. 


  How can i get my Money Back?


You likely cannot. Bitcoin transactions are not reversible, your only hope would be to find the person who stole your coins, and ask for them back. 

Sorry for your loss :(