pc will not power up

pc will not power up, I have removed the cover, when the power is turned off the CPU fan moves a little bit and a yellow LED blinks once, what is causing the pc not to power up, its was put up in a corner because it will not power up, any way today I pluged it in and it powered up, but I turned it off again so I could connect the monitor and key board and mouse, any how it will not power up again, I ve tried to bridge the power button terminals….nothing???