iMac startup issues

My iMac recently started freezing up until the point of where I had to turn it off but now when I start it up, it does not make the startup chime, the fans start to make noise for about 1 second and then the computer shuts off. The computer is "off" for about 3 seconds then the computer seems to startup again and the fans start to run but the screen is black, I have tried restarting it over and over again. I've left it on this black screen for about an hour and it does not change. Yes, I know I did not say the model becuase I currently do not know what it is, I do belive it's 10.7.5 though.