Have you had the side off the computer and checked if it has lots of dust and fluff on the insides. It might just need a clean. Disconnect from mains power and use a paintbrush to clean and a vacuum cleaner nozzle to suck up the dust that you disturb. Do not let the vacuum cleaner nozzle touch any of the parts of the motherboard in case of damaging the electrics parts.
When you have got it clean, reseat the memory ram sticks (take them out and then put them back in) sometimes they work a bit loose after a few years. Make sure all of the other the cables and connectors are firmly attached.
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