I had installed Ubuntu 12.04 on a separate partition, in ext4 format in my comp, which initially had only Windows 7. I had done the installation from a Live USB, which gave me the option Install on boot, in a GRUB menu.
I had chosen the option of "Installing Ubuntu alongside Windows". I had chosen the particular partition during the install, and in mount part, I had given /
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And I chose my only hard disk drive as the location to install the bootloader. The whole install process was over, and I was asked to restart, and while doing to it asked me to remove my installation media, which in my case was a Live USB.
I acted accordingly and removed it. While booting however, it booted directly in Windows, and no menu or option came for me to choose from both.
Best Answer
Maybe there is a Windows recovery environment (just maybe) and you chose that place as the place to install GBUB.
Whatever reason, follow these steps to restore GRUB.
Keep in mind that it doesn't overwrite the MBR, instead it adds a GRUB entry to the windows Bootloader.
Source
That's it! You should see a GRUB entry in the Windows boot menu when you reboot. Select it and choose "Ubuntu generic . . . . . .pae" from the GRUB menu(if the installation failed then you might not see it).
Good Luck!