What is a pity! If you have lost your BTC (lost your BTC wallet address & password) into someone else's hands, it is very difficult for you to retrieve them. In the lucky case, if that bad guy has not yet transfered BTC from your wallet, you can create a new wallet and transfer BTC to there. As far as I know we cannot change the password of a BTC wallet.
In case you lose your account on the exchange, let try to recover your account by email & QR code.
If you lose a lot of BTC, you can track their exchanging history & sue the thief.