Cannot boot ubuntu after fresh installation

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http://askubuntu.com – I installed Ubuntu 12.04LTS yesterday, with a live USB (ceated with Lili). I had windows 7 installed in the lap. I only wanted Ubuntu in my lap so , i gave the option for "replace windows with ubuntu". But after installation i am getting a blank screen (black) ,and the lap does not load any OS! So I tried reinstalling by booting from the live usb again, but what i got was the start screen of ubuntu! Its like now whenever I want to load the OS i need the USB to do it! Thank You for the help! (HowTos)