My MacBook Pro is on but the screen is black

Hi everyone, today when I was on the Internet my macbook froze so I held the power button for 5 seconds to turn it off but all that happened was the screen went black but the computer is still on (the sleep light is on and you can still hear the computer running) . I held the power button again but nothing happend. Has anyone had a problem like this before or know how to fix it?