computer turned off by itself. No picture on monitor

I just got back a computer that I assembled for my friend half a year ago. At that time, his motherboard went bad. So, I just replaced a new ASROCK motherboard that has most components build-in. (VGA, LAN, Sound) It was the last motherboard brand/model I could find in the market at that time since it's for Pentium 4 Socket 478. (I usually goes for Intel chipset after I found a lot of problem when working with SiS or VIA chipset.) All work well until a couple days ago. I got a call from my friend said that the computer shut down by itself twice. Later on, he tried to turn on the computer. The power on indicator shows up but no picture on monitor. So, I told him to bring that to me.

I've tested it with another monitor, no picture. The power on light stayed on as my friend said. I thought that it maybe because of power supply, so, I replaced it. It came back to work but without harddrive detected. The BIOS didn't see harddrive. Once I took a look at harddrive, I found that the cable was plugged loosely. So, I just pushed it firmly. The computer came back to work. Then, I tried to put back the old power supply. The computer still works.

So, the only issue I got here was about the IDE cable plugged to harddrive loosely?