Why will my custom built PC not turn on?

I had a PC built by my brother two years ago that worked fine up until this morning.

It has three LED lights. Two are always on while there is power to the computer, and one is on when the machine is on. The two that are always on, are on right now. When I try to turn the computer on via the power button, the blue LED flashes, the fan starts to spin then they both shut off.

I have taken the cover off and had a look, but nothing is out of the ordinary.

Does anyone know what could be going on?