Ubuntu fail to start on a dual boot with Windows 7 on Vaio

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http://askubuntu.com – I followed the instructions for the Ubuntu windows installer on my Sony Vaio laptop (model SVT131A11M). Upon restart I was prompt to choose the OS for boot (Win7 or Ubuntu), but then I shut down the machine and from the next boot, every time I choose to boot into Ubuntu, the GNU GRUB terminal appears with a grub> prompt. Now I'm able to boot into Win7, but can't boot into Ubuntu properly. Why does it happen and how come I was able to start Ubuntu successfully the first time? (HowTos)