I think after water is spill onto the motherboard, the power is shut down not only to prevent damage and short circuits but also it actually cause other components to go bad. So your pc does not shut down because it detects damage. It's because it can't run and all your chips and components like resistors and capacitors are damaged. So the screen goes blank.
I have done this before in a dual core pc. The screen froze and the mouse was disabled. After 2 secs the pc crashed and shut down. That's due to short circuit.
As far as i know even if you cool it after that incedent, there will be no such thing as bringing it back to live because a short circuit is damage to compoenents so there is no way reviving them and making use of them. Keeping your MB in rice or in the air conditioning for 3 hours will just dry the water but the damage cant be repaird. HOw can that be? If you damage your MB. I dont beleavew it comming back alive