Airtime disappeared

I'm a mtn subscriber and buy my airtime when needed. On Wednesday 23 September 015 I loaded R60 worth of airtime. I didn't make any calls or used internet and the airtime disappeared in minutes. On 24 September 2015 at 06h53 I loaded another R25 airtime. Although I had 249MB left I made sure that all contents were deactivated. At around 10h00 the airtime disappeared. I tried calling mtn call Centre but no one answered. What I really would like to know. What the F*** happened to my airtime! Is this your way of *****ing from your users? I never had this problem before because I've been using this number for many years. I loaded R40 of airtime yesterday evening, and my airtime is flying even when I'm not using my phone. I need an urgent explanation.