Mtn's very bad sevice.

Hi I have made an extra payment in February on my Mtn contract and every month my account showed that I was in a positive and all of a sudden that positive money just disappeared last month and I am trying to sort it out with Mtn via email and phone with no success. I do not know what to do or how to explain to then anymore. Please can someone help me. Mtn is just ignoring me now and I struggle to get any assistance from them.