Cell C

My grandma took out a contract with Cell C about 20 years ago contract ended and her number was now on prepaid. She lost her phone recently when we go to Cell C to do a SIM Swap she is told to go back to the shop she took out the contract at as nothing is picking up on their system. I mean that number she has been using it her whole entire life and why is nothing is picking up on their side?? How do we go about in getting her number back?