Cpu turns on slightly and restarts but slightly again

Hi.
When i turn on my pc my power button has lights where ican see whether it's on and fan is open on the upper side.
So this time i pressed my power button it starts to lightup and off back again and it happens again and again so i looked into my fan and i amsaw that the fan turns on and off i tried other cables other avr but nothig seems workin
hope youcan help