My airtime disappeared

I had airtime worth R280 on Sunday and around 200MB data. I made a hotspot and connected my laptop for like 10 minutes and when I checked the airtime after using the internet I had R0.00 and 6KB data. I really don't understand how that happened because I was just on Facebook, I was not downloading anything. I called the call centre and the answering machine said the call centre is closed. I called again on Monday during working hours and I was cut while holding more than once. I don't know what is happening with MTN, I'm getting fed up. I'm due for an upgrade in December but I don't see it happening. Can you please explain this.