Your contract is with the mail order catalogue who sold you the item and not with Hi-grade.
You should therefore:
1. Contact them immediately, and make clear that you expect them to put matters right under the Sale of Goods Act.
2. If you are, as I assume, still making payments to the catalogue for the laptop you may want to make this clear when you contact them.
3. If the catalogue still maintains misuse make clear that you will require that the laptop is made available for an inspection by a computer engineer who will check what has caused the failure. If you have not caused damage then the catalogue will have to pay for both the repair and the cost of the inspection.
It is vitally important that you deal not with the manufacturer, who has no contract with you and so is not covered by the Sale of Goods Act 1979, but with the seller who is. Act very quickly and if you phone initially make sure you confirm everything in writing.