Problem with my PC

[Xx_DonutEater_xX]

So yesterday I changed the cooler for my cpu. I installed the hyper 212 evo, but bow my pc won’t boot. I don’t know what I did wrong. This pc is about 5-6 years old, but it still worked fine and it would be great if I could use it again. By not booting I mean the PC is turning on, fans are spinning, but this only continues for 1 second. Than the PC turns off. After that it doesn’t turn on automatically. It turn on when I press the power button and turns off in 1 second.

[Cuirl]

Did you apply thermal paste? Silly question, but if you didn't then it'll automatically shut off as a security measure. 

[Xx_DonutEater_xX]

Yes I did. But on that note, I had to work on this exact pc for couple of weeks without a thermal paste, so it will definitely work even without thermal paste.

[Cuirl]

A CPU running without Thermal Paste? You might have damaged it if it's been running hot for weeks with no thermal paste on it. 

[Xx_DonutEater_xX]

No I haven’t damaged it. I have undervolted my cpu so it didn’t run hot. But after that I had thermal paste for about 1 year and it worked perfectly fine. I didn’t do any heavylifting with that cpu without thermal paste.

 

[Cuirl]

Well, if it turns on and off after a second it's likely a safety measure thats built into the motherboard to prevent it from damaging anything. That's the only thing I can think off. But if you say thats not the case, then I can't help you sorry. 

[Xx_DonutEater_xX]

So what can be causing this?