PC Suddenly shuts down then won't turn on again.

My PC was on for about 3hrs today morning. I installed a software and the PC shut down immediately and is not starting up again.
Specs are in my signature.

I've opened it up and saw when I turn the power switch on the motherboard standby LED lits up. However pressing the turn on key does nothing. Also the mouse lights lit up.

Any help would be appreciated.

You can 'try' the reset button. However, this seems more like a failed PSU. IF you have another PC the borrow the PSU and attempt to start the PC. Generally, a failed PSU gives some sign of its' failing before going 'poof' but not always.

It might be worth the time to also try a Jumper Clear CMOS - see your manual.

You can even have the fans spinning with a failed PSU, and sure the LED can light-up like a Xmas tree. However, IF you have e.g. RED where RED suggests a problem, see manual, e.g. bad RAM, bad CPU, bad GPU, etc then you could have a failed component.

Re-seating the CMOS batter is NOT resetting the BIOS anywhere near the effectiveness as using either the Jumper method nor CMOS Clear button. IF you pull the CMOS battery then you must also unplug the A/C and allow the PSU to fully discharge.

IF your animals dripped water into the case itself laying on it then sure 'water' as far as humidity and temperature it would have to be so bad that you'd feel absolutely miserable.

IF your animals chewed ANY of the cables then YEP that'll cause a post failure. To Test: note the USB locations to USB Ports {put back later in SAME place}, disconnect ALL external cables and 'try' the power button. I've seen a tooth-pierced Mouse cable cause a major problem.

Otherwise the place I'd first start is the PSU. You can try a 'Breadboard' as follows:
You can try Lazyman's Breadboard -
* Disconnect ALL Front Panel & USB Headers ; keep only the PWR & Ground <best> short w/wire.
* Disconnect ALL peripherals including the Keyboard/Mouse
* (1) Stick of RAM in the second DIMM slot from CPU
* Unscrew both the MOBO and PCIe screws

Pull the MOBO away from anything conductive and supported by a towel ; short PWR & Ground to start.

If the problem persists then do a FULL Breadboard {removal from case}, IF your HSF has a metal backplate with no conductive insulation e.g. plastic washers then: Screw -> | Backplate | Plastic Washer | [MOBO] | etc...

As stated...Otherwise the place I'd first start is the PSU. You can try a 'Breadboard' as follows: followed by the remaining items listed. I've seen plenty of PSU posts, but in order of failures: PSU, RAM, GPU, MOBO, and CPU. IF you're already using the onboard GPU then scratch it off the list otherwise it's easy to pull the discrete GPU and use the onboard 'VGA.'

You have a guessing game at foot, and I'm playing the odds and the symptoms. Note what I've already stated. TRY a PSU or waste HOURS screwing around with whatever you want.

You can simply unscrew the MOBO screws and pull away the MOBO from the standoffs and attempt to start.

At 13 you're plenty smart

I've just tested the PSU on another computer and that is working fine with the PSU.

I have took the MoBo off the case and placed it on top of a towel and still it didn't boot. Any idea?

I took it to the shop and after testing the parts they've said the MoBo is dead.
I took the MoBo to an Intel RMA center and submitted it there.
They said one lot of MoBo's with H61 Chipsets have problems. They also suggested me to change the PSU unless I want to come to them again.
It'll take anywhere near 7 to 30 days to get a new board.

Hmm..
So most probably its the PSU which killed the MoBo.