I recently applied the latest Windows 8.1 patches. To my dismay, I found that the core Ubuntu 14.04 entries at boot time were missing and by default, the laptop always booted into Windows 8.1.
This meant that my Ubuntu EFI boot partition had been corrupted or taken over by Windows 8.1 patches that probably updated the boot entries.
To confirm, I downloaded the Ubuntu Live CD and booted into Ubuntu Live.
In Settings -> Gparted, I could see the Ubuntu boot partition was intact on sda7 and the default boot partition was something that Windows had hidden away from Ubuntu.
Next step was to check and confirm observation with Boot-repair. Boot-repair utility will diagnose the boot partitions and then try to repair it. My first goal was to boot into Ubuntu and backup my data. After that, I would not mind experimenting. However, not much choice, so did a boot-repair and followed the defaults shown on screen when it did it’s magic. Below are the instructions to run the boot-repair program from a terminal window:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair
When the boot-repair application is started, click on Recommended repair.
Finally, it said the default boot option was Windows and it had now changed it Ubuntu.
Restarted the computer and was now able to login to Ubuntu and take a backup.
If you are paranoid about something going wrong, take a backup of your disk using clonezilla or any imaging tool before performing a boot-repair. No one is responsible.