MTN number deactivated

To whom it may concern

I had an MTN number for close to 20 years. I have been travelling abroad for the last year with my MTN sim card being used in my dual sim mobile to keep it active. I will be returning to South Africa later this month and i now see that the sim is no longer working so i am assuming that it has been deactivated by MTN. How can this be done when the sim card is still "active" in a dual sim phone and there is still credit that wasnt used on the sim. I am still using this number for whatsapp and need this number reactivated