Something really wrong with my pc. it wont turn on. what wrong?

well a while back i turned my pc off from the main power button. it did turn off but my usb flashing mouse was still flashing. i tried turning it on again and nothing happened. afterwards i bought a new motherboard thinking that might fix the problem. it didnt. i just bought a new cpu and it still wont turn on. but... show more well a while back i turned my pc off from the main power button. it did turn off but my usb flashing mouse was still flashing. i tried turning it on again and nothing happened. afterwards i bought a new motherboard thinking that might fix the problem. it didnt. i just bought a new cpu and it still wont turn on. but when i put the mains lead in the back of my pc the usb mouyse starts flashing again. what is wrong with my pc?
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