MTN refusing to change my number to a prepaid number

MTN has stopped my number from working only cause my 24 month contract had come to an end and i wanted to change to prepaid. This is a number I have used ofr 12 years.... countless emails have been sent, for two weeks now the world cannot contact me. I cant do my business. My workplace has threatened to discipline me cause I dont have a line to be reached on when Im on standby.