Blocked sim without reason?

For the past 3 weeks I have not been receiving calls and I cannot call people either. I thought it was maybe my phone but I saw that the problem is related to my sim. I called MTN customer service 4 times and I dailed what they said I should dial but nothing was fixed. I called again and they said that they are not sure what seems to be the problem. I called again and this time I was told that my SIM card was blocked and I was not even given a reason as to why. Till now my sim is blocked I cannot make calls nor receive calls it just goes straight to voicemail without it even ringing.