If you desperately need to turn on the computer, you can use some thin metal such as a screwdriver and slowly slide it across the front panel pins. You might eventually find the two pins that are meant for the power switch. It should also be labeled, often hard to read, under the rows of pins.

Finding the model of your motherboard is easy. It is usually printed on one part of the board itself. Most of the time, they are next the the memory banks.

You can then find a diagram from the motherboard's manual. They will often be available for download on the manufacturer's website.