Computer Turning On But No Signal

A couple months ago I helped a friend build his pc (I have experience and my own custom built ones work flawlessly). After a couple of weeks for some reason most of the time he turns his computer on the fans are running and the hard drive is spinning but the monitor, mouse, and keyboard get no signal. He will wait for a couple of minutes and nothing happens. He then has to turn off the computer remove the graphics card and ram and replace it back and hope it works. If it doesn't he repeats the process. Is there any reason why this is happens. Could the psu be bad. When he bought it i made sure to get a very good cooler master one with great ratings and 80 Plus Gold. Please help, I feel bad because I convinced him to build one instead of buying one and now it doesn't work properly. 


Thanks in advance,



power supply is dying