Heeeeellpppp

Hi everyone, im new to joining macrumors but ive often come here for info on problems im having.Well this is a new one for me.So i was using my mac pro and it just froze;i got the wheel and nothing was responding i had tried installing and opening a torrent of osx mountain lion (i know stupid me ) shortly after thats when it froze .so i held the power button to force shut down. go to turn it back on and i get the prohibited symbol.
PLEASE HELP !!:eek:
Thanks !!