HDFC Bank — Unauthorized payment transaction without any OTP

Dated: 14th August 2020 Time: 5:50 PM

I suddenly received an SMS from my HDFC credit card that a transaction amount of Rs. 748.75 has been deducted from Amazon Digital. I quickly logged in to my Amazon account and found that there are no new orders, however, the above mentioned amount had been deducted from my HDFC credit card account. There was neither any OTP in my registered mobile number nor in my registered email address.

The SMS that I had received from HDFC credit card department reads like this "ALERT: You've spent Rs. 748.75 on CREDIT Card xx8858 at AMZN Digital on[protected]: 17:50:25. Avl bal - XXX ..."
SCREENSHOT attached below with this complaint.

How can a transaction go through successfully without any OTP generated in my registered mobile number / email address? Please suggest and advice me what should i do next and how do I get my money back?

Regards,
Mithun Paul.
Registered email address: [protected]@yahoo.com
Registered mobile number: [protected]