Why is my mac screen black?

My mac would not turn on from sleep mode the other day so, as you do, I restarted it and the screen stayed black. I could tell the computer itself was on because it made the correct noises when restarting and made noises when I pressed the keys on the keyboard. I've restarted it several times and the screen is still lack. Please help me, I don't have any clue what to do? I believe that I have the iMac G4 but I cold be a few models off. I'm not quite sure what operating system it is on but I've tried to make a guess. Thanks.