My bitcoin is stolen and following is how it happened? A guy named, "David Robin" from Fremont,California US (obviously a fake one), connected in a social network told me that I can mine for bitcoin when I connect my blockchain wallet to the blockchain mining network with the help of a antminer s9 hardware. He asked me to register a blockchain account and send him the wallet id and password so he can do the connections. He also said that once he is done with the connections, I will change my password. I shared with him my user name and password information. He did something with it. I changed my password and then topped up the Blockchain account with 0.198899 BTC, approx. $3,200/- on 27/12/17. Within half an hour my wallet was empty. I will appreciate if someone can explain how it has happened and if there is any way to get my BTC back?
I'm sorry you were scammed. One of the reasons that many experts don't think bitcoin is ready for mass adoption or retail use is precisely because people don't understand how to keep them secure.
You gave someone access to your key and they made a copy of it, then you locked out their access to your key. That didn't do any good though, because they had already made a copy.
Changing your password prevents anyone from using that password to access your wallet and keys. It doesn't prevent anyone who already knows your secrets from just creating another wallet with the same secrets.
About your only hope for getting the money back is tracking down the scammer.
Sorry to say, you were scammed.
If you sent him your wallet id and password, he could log in and copy your seed/private keys. So it wouldn't matter if you later changed your password, he could recreate your wallet on another device, and wouldn't need your password to access it on that device.
You'll have to contact the scammer to ask for your btc back, otherwise they're gone.