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Unplug anything you have connected except the mouse and keyboard, internet connection. Try to boot it up again. If it won't get past that try starting up to the install disc or a safe boot. Run Disk Utility and verify the disk and disk permissions and try to reboot once more.
 
had the same problem

what I did was I read my manual that got off the apple.com
and I did all trouble shotting like rebooting the RAM and reparing HD

I ended up re-installing my software and luckly I had everything backed up using time machine.
 
ive had this happen when Updates havent installed properly,
just boot off your system disk and run disc utility, you may have to reinstall.

if you have another computer handy, and if its a mac you can network boot...
plug in to LAN and hold N when booting up. you can then access it through your finder from another mac with password and username. i'd back up allmy files and re-install mac osx. sometimes the easiest way.

im sure you can do similar with another PC but i dont have any knowledge in that
 
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