PSU replacement not working properly

Hi there,
I was playing on my HP Omini 220 desktop when I smelled burnt plastic, on investigation, I saw fumes of smoke coming from my Power Supply Unit, I quickly unplugged my computer from the wall socket without switching it off, I opened the casing and saw a smoked fuse, I replaced the PSU with CORSAIR750 watts, the same model as the previous one, the motherboard LEDS are going on well but the computer won't boot, what could be the problem and how can I fix this?
