PC shuts down without warning.

I have built my custom PC about 1.5 years ago. In April my Computer shut down randomly without warning, I wasnt doing anything different than usual. I didnt think much of it at the time.

About 1 month later the exact same thing happened again. I researched it but found nothing that fit it exactly. Then it happened again approximately 4 weeks later. I then decided to clean my PC in case it was a thermal issue.

Now, 1.5 months later, it happened again. It once again had no warning, it just turned off like someone unplugged it. I can then turn it on again like nothing happened.

Does anyone have some ideas as to what could be causing it? Thank you in advance.