Standard Bank *****ed my dad.

I recently had an interaction with Standard Bank, reporting my dad's account, which went dormant after he died in 2005. I had no idea he had an account with them at the time, but we also learned recently that he had an investment with the bank before he died. I wrote to Standard Bank and went on to enquire with them about the status of the two accounts. I found out that there was no record of either of the accounts ever existing with them. I gave them documentary proof of the accounts and they could not find anything. What is odd is that their staff conceded that the profiles are never deleted, they live longer beyond the owners. All my efforts to get these monies back for my 85-year-old poor mother drew a blank. How does a blank as big as Standard Bank lose such crucial information? If there is proof that the account these accounts existed, how can they dismiss our case only because the account is 20 years old? My father invested his last R22000.00 at the time and it just disappeared, just like that.