Most banks have staff dedicated to matters like this. The relevant contact details are found on the backside of your card as well as the website of the bank. Telephone numbers of help desks are also displayed at every ATM machine.
If you have suffered a loss you must immediately contact the banks via phone (preferably) or email. Do not forget to note the complaint number and follow up your complaint using the same number. The bank should acknowledge your email.
The Code of Bank’s Commitment to Customers (CBCC) enacted by the Banking Codes and Standards Board of India (BCSBI) mandates each bank branch to display the name of the official at the bank branch responsible for addressing customer grievances.
If your complaint is unresolved at the branch level, you may approach the Regional or Zonal Manager or the Principal Nodal Officer (PNO) at the address displayed at the branch.
Typically, within 30 days of receiving the complaint, the bank will send you a final response or explain why more time is needed to investigate. The Bank will also tell you the process to take the complaint forward in the event you are still unsatisfied after the final response from the bank.