http://www.howtogeek.com/114884/how-to-repair-grub2-when-ubuntu-wont-boot/
This one I have tried and it works.
Boot from a live CD (CD/DVD or flash drive).
Ctrl+Alt+T to open a terminal window
sudo apt-add-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair
boot-repair
From the web site:
The Boot Repair window will appear after you run the boot-repair command. After it scans your system, click the Recommended repair button to repair GRUB2 with a single click.
You can also click the Advanced options header to customize GRUB2’s options without learning its command-line syntax.
Restart your computer after using the Boot Repair tool. Ubuntu should boot up normally.
Someone on that site says, though I haven’t tried it:
Just open gparted , right click the partition that has Grub in it, go to manage flags and mark boot.
well, that last bit didn’t work for me but I’m leaving it here for posterity.
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