If they didn't tell you what killed it, don't believe them. Go inside the case and see if removing the RAM sticks - one by one if there is more than one. While you're in there, check that the fan doesn;t stop after a while. That would cause a failure and ultimately would kill the CPU.
Ideally, you would download the manufacturer's service manual but if you can't find one, you need a 2.5mm Pozi screwdriver, a magnet and an old credit/hotel key card. When you remove screws from the bottom and they differ in length, put them on a piece of paper on which you've marked where the holes are in the case. If you put a longer screw in where a short one should go, it can wreck things in seconds.
The magnet stops you losing screws and the credit card is to get into the base from the top. Using a knife is a bad idea.