After +16 years number re-allocated

I have been with MTN for over 16 years, first as a pre-paid customer then contract and then again as a pre-paid ***). When I changed from contract to pre-paid I told the assistant that I am out of the country for several months but I need my number as my whole life is attached to the number. She assured me that it would be fine. During the time that I was away, I tried to send an SMS but I was unsuccessful. I made sure that I loaded air-time onto the cell before I left. Cutting a long story short, needless to say, my number has been allocated to someone else. How can this happen!!! I am so angry with MTN. Another point, when my husband passed away, I took his death certificate to the MTN offices and ask if I could cancel his 2 contracts and they refused to do it. I paid the accounts for 2 years and this is what they do to a person. I lost money when I was on contract and now I seemed to have lost again. !!!!! The money is not really my concern but my number is. I would really like my number back!!!!!