BILLED MORE THANT DOUBLE MY NORMAL DEBIT ORDER AMOUNT

I HAVE 3 CONTRACTS WITH MTN -

1 BUSINESS CONTACT EXPIED IN OCTOBER UNKNOWN TO ME AND IN NOVEMBER DEBITED ME R2600 MORE THAN MY USUAL AMOUNT I WAS TOLD THAT DUE TO THE CONTRACT BEING EXPIRED I WAS BILLED NORMAL RATES THIS IS UNFAIR NO ONE PHONED ME TO ADVISE I MUST UPGRADE AND IF ITS EXPRED WHY DID IT NOT STOP WHY WAS I BILLED THIS AMOUNT I WANT MY REFUND AND AND SOMEONE TO HELP ME WITH AN UPGRATE