IMHO, The best way to attack a problem like this is to divide and conquer
and to minimize the variables as much as possible.

If what Thrax recommended doesn't work......

Here's what I recommend:
open the box

Note - Before Proceeding:
Be mindful of taking ESD (electrostatic discharge) precautions before
touching any components inside your computer.
(http://www.computerhope.com/esd.htm) The potential for
damage to your sensitive electronic components from electrostatic discharge
is very real. Your body should be at the same "potential" as the devices you
intend to touch b-e-f-o-r-e you touch them. If not, an (invisible - you
won't even know it happened) ESD will take place when you touch it and
possibly damage your device.

Remove all CCAs (circuit card assemblies) not necessary for the system to
function. Usually the video CCA (if you are not using integrated video) is the
last remaining CCA.

Disconnect all unnecessary devices' power and data cables (optical drives,
floppy drives, etc.) Reseat all remaining cables, etc. Reseat memory and
CPU.

Take your memory down to the minimum no. of modules (a single module or
pair depending upon your type of memory)

** Remember your ESD precautions especially when handling memory.
Handle the edges of the modules only. Don't touch the ICs ("chips") or any
circuitry on the modules. **

Disconnect all devices connected to your computer externally (printers, usb
devices, firewire devices, etc.). You should be left w/ just your mouse,
keyboard, and monitor connected to your computer externally.

You should be down to your power supply, motherboard, RAM, CPU, video
CCA (if it isn't integrated) & hard drive. IOW: The idea is to take the sytem
down to bare bones. Only hardware that is absolutely essential for your
system to P.O.S.T. (go thru Power On Self Test) In this way, you've
eliminated as many variables as possible.

Now, try powering it up. If it boots, start backing up - systematically adding
back devices until you find the issue. In some cases, you may find that the
problem doesn't return. You get all the devices re-installed and the system
is ok.

If it still doesn't boot, consult your Asus mobo manual for directions re: how
to clear your CMOS. Once you've done this, try booting again.