I was told my account is in arrears

On Friday 16/10/2015 I went to Vodacom shop to renew my contract only to be told that I cannot because my account is in arrears by R1074.00 which I find very strange. Every month I pay my account in full and I do not miss a cent. The only time I did not pay in full was when I received credit from Vodacom for charging me for a wifi router I had returned back to them. Not even once did I receive a call from Vodacom regarding this long outstanding amount. Even when I go to my MyVodacom It does not appear. I even manage to do an upgrade last month in September without being told about this amount. Why now? I hope this does not affect my status. I am very troubled by this.