Motherboard short circuit

I have a desktop PC which is mostly used by my younger brother as I am used to my lappy only. Last night he opened the case and tried to clean the parts that had loads of dust on it. After cleaning and all, he placed everything properly and after that the computer will not start.

When checked inside the cabinet with machine ON, it shows that PSU is ON, CPU fan running but there is no power to the HDD and no LEDs glowing and therefore no starting of the PC. So that means only the motherboard is not ON.

Is it that the motherboard is short circuited some where? What can we do?

Make sure that the motherboard is not short circuited at any visual points or to the cabinet metal parts. Quick solution will be to place a plastic sheet beneath the motherboard and make sure the cabinet's metal cover is not in contact with the motherboard.

I think the only thing that can trip a breaker is a bad PSU. I would consider replacing that first, or at least swapping it temporarily. A multimeter may be helpful if you jump it. Best to have a load on it when jumped for any length of time.

If the fans come on and shut off quickly, this might be it. Just to be sure, you talked about having everything plugged in, this does include the 4 pin molex, doesn't it? If there was any smoke issue, then it is not a good sign. Check for capaciters that are bulged or cracked, check for possible dark spots that might indicate a heat issue someplace on the board that might have shorted....a good vidual inspection can sometimes tell you a lot.

Have you ever had any issues with your monitor at boot. i.e no display until an OS logon screen appears. This only happens when using DVI.