Why won’t my PC turn on after I shut it down?

I've had my PC for about two years, but recently it's been acting up. A month ago, it just wouldn't turn on at all. I took it to a repair shop where they did some cleaning, and it booted up again. However, the next day it wouldn't turn on after I shut it down. I brought it back, and they tried using a different power cable. It worked for a few days, but then it had the same issue again. The repair shop even did a home service and found that there was no power coming from the PSU when they tested it at my house. They took it back to the shop, and it worked fine there. I brought it home and it seemed okay for a while. But when I tested it again by turning it off and on, it wouldn't power on at all. My dad had a friend check it out, and he said it might be a PSU issue; it worked for a bit afterwards, but now it's back to not turning on. I've tried a few things: using only one RAM stick, plugging it into a different outlet, resetting the CMOS, and changing the power cable, but nothing seems to help.