Why was giving someone else my 12 word backup a bad idea? I lost my bitcoin last night when someone on blockchain support asked me to tell them my 12 word backup in order to reverse my bitcoin transaction that was unconfirmed at that time. Finally, my balance remained zero. Can someone explain to me what happened? Is there any chance for me to recover my funds?
someone on blockchain support ask me to tell him my back up fund 12 word in order to reverse my bitcoin which was unconfirmed in my transaction


Sorry to say, you were definitely scammed. 

You cannot reverse a bitcoin transaction in that way, the scammer only told you that so that you would give them the 12 word phrase. With that phrase, the scammer could recreate your wallet on another device and steal your coins. 

Do not ever give someone your private keys, seed phrase, PIN code, password, etc. Any legit customer service will not ask you for this.