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I upgraded my Windows from 8 to 8.1 just now, then when I tried to log in to Ubuntu, I didn't get Grub, instead the following message appeared -
error unknown file system grub rescue>
Now, I logged into Ubuntu using Live USB and tried running boot-repair, but it gave me the following error -
An error occurred during the repair.
A new file (~/Boot-Info_2014-05-04__21h37.txt) will open in your text viewer.
In case you still experience boot problem, indicate its content to: firstname.lastname@example.org
You can now reboot your computer.
You may want to retry after deactivating the [Backup and rename Windows EFI files] option.
The boot of your PC is in Legacy mode. You may want to retry after changing it to EFI mode.
Content of .txt file is here - http://pastebin.com/L5ZNCTRe