Huge Data Bill from Nowhere!!

I am highly upset with MTN really. I had about 1.2GB left of Data during the second last week of July 2015. My laptop wasn't being used much, it's at home during the day, of which nobody is there. Its off most of the time meaning my modem is not in use. Suddenly on the 1st of August I could not access the net. I kept on getting an error, I could not even check the remaining data. And I still couldn't connect to the Internet. I managed to call the contract call centre on the 6th of August 2015 as I had not received feedback on the email I had sent to customer care. The lady I spoke to, said my SIM had been barred. When I asked why, she said she didn't know. So she unbarred the sim card. I used the Internet briefly on Saturday to check mails and a few things and that was it. The laptop has not been in use much and has been off the whole time and the modem packed away. Yesterday when I logged onto MyMTNActive to check my data balance, I saw under unbilled invoice, a bill of +-R2200 which I don't understand because nobody had been using the modem so how do you explain such a huge bill?I need this explained to me, I'm not paying for this, for something I didn't use. its ridiculous.