the pc cpu doesn't work

CPU=Central Processing Unit or otherwise known as your Processor.
PC=Personal Computer

In your case there is a problem with your PC and we can assume it is a Tower PC or Desktop PC

If you have a lack of experience and your machine isn't powering on, a forum might not be the right place. You might want to bring it to a professional. Think of a computer like a car. If you call a repair shop and say it doesn't work they are going ask you what doesn't work and tell you to bring it in. They aren't going to walk you through fixing your engine over the phone. However, if you can be a little more specific than "doesn't work" we can see what we can do. You cannot make the assumption that your power supply and motherboard are good until they have been tested properly.