my mac is not booting and show an screen with no enter sign

My iMac - S/N:C0*******NCR HAS TOTALLY FREEZED. THE OS X MAVERICK HAS CREATED HAVOC AFTER I TRIED TO INSTALL BOOTCAMP. NOW WHEN I BOOT SCREEN SHOW APPLE AND AFTER 2/3 MINUTES IT SHOWS NO ENTRY SIGN. I HAVE CARRIED OUT HARDWARE TEST RESULT IS NORMAL, BUT STILL IT DOESN'T START. WHAT CAN I DO?ALSO COULD SOME ONE HELP ME TO REMOVE OS X MAVERICK AND LOAD MOUNTAIN LION.

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