I have just built my own PC a week ago and I used a secondhand Seasonic X 650W which I got from a friend. It works fine for the first day and used it for about half a day but something weird happens after that. When I shut down my PC for the first time I didn't unplug it from the extension with built-in surge protector and I also don't turn off the PSU. The next day, switching the case power button won't start up the system but after random workarounds, flicking the PSU off then on again seems to solve the problem and I would go on and use it for hours. I also tried shutting down the PC and using it again after about an hour and it seems to work fine then. But when I use it the next day, the same thing happens. Now I'm suspecting that turning it off for long hours may be causing the problem. I lived with it for about two days and it's really annoying to have to do the workaround everyday. But today, I used the same workaround and got the PC starting but it shuts down automatically after about five minutes. I tried unplugging the cord and discharging the capacitors by holding the case power button and when I try to turn it on again, it always shuts down automatically.
I don't think it's the motherboard since I can boot and use it fine before it shuts down automatically. I suspect that this is because of a faulty PSU but my friend said that he never encountered this problem before. Can you help me troubleshoot this? I would really appreciate any feedback because it's really frustrating.