How can we legally recover unauthorized transactions from my mother's account?

My mother operates a distribution agency and holds a current account with Union Bank. Recently, she left our hometown of Thorrur to travel to Thiruvannamalai. Soon after her departure, we were unable to reach her by phone, as her number appeared switched off despite the phone being on. Initially, we ignored this, assuming the SIM had failed. Upon her return to Thorrur, my mother obtained a new SIM card with the same number. Shortly after, we received a message indicating that 9,000 INR had been debited from her account, which my mother did not authorize. We immediately visited the bank and froze the account. Unfortunately, we discovered that a total of 2,71,000 INR had been lost through approximately 40 unauthorized transactions. These transactions included payments via PhonePe and transfers to various digital wallets, none of which my mother authorized. We did not receive any OTPs or transaction notifications.