iMac 27 inch- No Boot and Blue Screen

Hi

I have an iMac 27 inch running OS X 10.9 Mavericks.

My iMac is not booting and here are the symptoms:

-I pressed the command+r key whole turning on the Mac

-I saw the apple logo and then a status bar and the circle thing telling me it's trying to do something in the background

-blue screen comes up after that

-turned of the Mac

-pressed the shift key while turning on Mac

-saw the apple logo and after a couple of seconds, the screen seems to be off but I know that's on

It's just weird because the sand exact thing happened to my friend 2 weeks ago, she's also using an iMac 27 inch. Did apple pushed an update to our computers recently or is this a known issue?

I appreciate it if you can give me a workaround for this problem.

Thanks!

April