Macbook doesnt boot after security update

[Alexmraposo]

Hi, I'm trying to help out a friend with a macbook problem. I'm a pc user so I'm not up to date with the easy fixes for apple and all so I figured I could try here. After she downloaded and installed a security update last night her macbook restarted and made "a hard drive sound or disk reading sound" but the screen was just black. I have tried restarting with the power plug in, tried holding down the power button, and nothing happens. I do hear the cd drive starting when I first boot and the screen does flash at first but just goes black there after. The power indicator light also stays on. Any ideas? Thanks in advance!