contract expired but I still receive demands

I was told that my contract expires on 9 Dec 2015. Giving me the idea that it will end automatically. Today, 04/03/2016 I received a notice claiming that I owe R699,88. Firstly, I was unable to use my laptop since sept 2015, some software problem which I had to pay for in order to have it fixed. I was told by customer service. Secondly, I was never told that I need to give notice to cancel my contract. It was a 24 month contract and that means it will and should end after the 24 months are over. I am not happy to pay this amount, the only amount that might be payable by me, is the Dec 2015 payment of R269. Please, I want this fixed.