extremely poor and unacceptable service

I bought a laptop from FNB in 2015. Lately the laptop keep on switching itself off and I contact FNB regarding the problem. They send a courier company to collect the computer and take to repair company in George. I received an email yesterday indicating that the laptop showed signs of trying to open it which voids the guarantee. They now want me to pay more than R19 000 to repair the laptop. I am still paying monthly instatements on the laptop and am using it in my work. I am not stupid to try to open a computer and then contact FNB regarding the problem. Just a fool would be so stupid. I contacted the complaints section of FNB as well as the repairing company regarding the matter. The only persons that could have tried to open the laptop can only be the courier company, as they have fetched the laptop from me, without covering and sealing it and took it away. There is no way that I will accept their accusations nor will I pay anything and I want the laptop repaired as it is still under guarantee.