problem with turning computer off and....

[b1gapl]

My bro's comp wouldn't turn on so I unplugged it and plugged it back. This time it did turn on but nothing on screen just blank. Now when I hold the power switch to turn it off it doesn't. The only way to turn it off was for me to pull the plug. Can someone please help? Also the only way to turn it on is to plug it in the outlet, without pressing the on switch. My bro's computer is like 7-8 years old, could this be a factor?

 

[TheMajor]

AT or ATX?

 

[b1gapl]

The mobo is a MicroATX.

 

[TheMajor]

Check if the small power connector is propery connected to the motherboard.

 

[b1gapl]

Small power connector? Is that the 4 pin? Because there is a 4 pin molex right next to the 20pin ATX power.