What to do if signatures have been forged after death

My Father had given a Collateral Security to an Over Draft of a Society in 2003. He never went to bank to sign anything later on. However, we found the Debtor was submitting the Letter of Guarantee(s) duly signed every year. My father died in 2014 and the OD was maintained even after my fathers death and a Letter of Guarantee was submitted even after his death. Now, another Loan of the Debtor in the same bank gone NPA. Now, a Notice was served a U/S 13(2) SARFAESI in the name of my Deceased Father, for the total liability of the said Loan referring to the Guarantor Letter dt. 3/7/2014. However, my father died on 12/06/2014. How should I proceed in this..