Yes. My mistake. It was a "HAIR DRYER".
The computer power on with the monitor not displaying anything. The monitor's power button is orange, meaning it is getting no signal from computer. The cpu fan is ON. The case fan is ON.
My only guess is that he might have "...generate voltage and fry something on the motherboard" or hair dryer might discharge static and got to the motherboard and now only some service can reach some of the components but not send signal to monitor. Also the power button led remain unlit even though the case fan/cpu fan is ON.
I test each component by removing it individually. I tested the RAM (2 pair - 4sticks. test a pair at a time), Graphic Card, disconnected optical drives, disk drives, BUT the same problem still lingers on. The first couple of days the computer would work like normal except having to power it on took a few press of the "power button" but it worked. Now, I can't even get the monitor to display anything, let alone I can't hold the power button for 10 second and power down the computer. Any ideas? Thanks.
Electrostatic Discharge can take out any circuit, but I find it unlikely to be the cause if all you've done is blow out the dust! It sounds like he tried to use a hair dryer?
There is always the chance that its coincidence that the PC is playing up now after cleaning it, just one of those things.