I had a dual boot setup on my Dell laptop with Ubuntu 16.04 and Windows 10. The tech replaced the motherboard and now I'm not able to boot. I am trying to restore GRUB - I installed a version of Ubuntu 16.04 on an USB stick, following the instructions on this site. Then I tried to boot from USB, but received this message:

Missing operating system

Selected boot device failed.

Can somebody tell me how to proceed?

Part 1 - Creating the installation media

3rd party installation media creation tools like Unetbootin are not creating the USB installation media properly in many cases. A tool to create a correctly working installation media is GNOME Disks, it's available in the repositories of nearly every Linux distribution.

Open Disks - select Restore Disk Image from the menu on the top right.

Choose the ISO file and the USB drive to write it to, then start restoring.

In case you have no access to a Linux operating system and have to do it from within Windows, you can create the installation media with the diskpart tool from a running Windows system.

Open Command prompt as administrator and execute :

diskpart

list disk  

select disk *  

clean  

create partition primary  

active  

format fs=fat32 quick  

assign letter=**  

exit

Note : * = number of USB drive | ** = select a free drive letter

Now mount the ISO file and copy the content to the USB disk.

Part 2 - Reinstalling the GRUB boot loader

Boot from the Ubuntu installation media - select the option Try Ubuntu without installing.

Once you are on the Live desktop ... open a terminal and execute the following commands.

Important information concerning the USB boot options :

Choose the USB entry with UEFI in front, in case the systems are installed in EFI mode and

the USB entry without UEFI in front, in case the systems are installed in legacy BIOS mode.

In case your computer has UEFI BIOS execute these commands :

sudo mount /dev/sdXXX /mnt

sudo mount /dev/sdXX /mnt/boot/efi

for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done

sudo chroot /mnt

grub-install /dev/sdX

update-grub  

exit

Note : sdX = disk | sdXX = EFI partition | sdXXX = system partition

In case your computer has legacy BIOS execute these commands :

sudo mount /dev/sdXX /mnt  

sudo grub-install --boot-directory=/mnt/boot /dev/sdX  

for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done  

sudo chroot /mnt  

update-grub

exit  

Note : sdX = disk | sdXX = system partition

You can identify disk and partition numbers by using GParted (included in the installation media).

Additional information : Do not forget to disable hibernation and Fast startup in Windows !

Then shutdown the PC completely, do NOT restart - otherwise it will always start Windows.

This is a different answer, as I've just had the same problem.

My logic if you just replaced the MotherBoard then your hard drive (including its boot sector) should be the same so those recovery steps shouldn't be necessary.

Part A:
Ensure that you have the correct boot type. Legacy / UEFI your error message above seems to imply you have the wrong one. Try changing this option

For me UEFI was the correct one.

Part B (UEFI only):
In bios find your boot sequence options.
Add a boot option (your bios should be similar to mine)
browse until you can find the correct EFI file for grub.
example:

• 
  Boot Option Name: grub


• 
  File System List: ~DID NOT CHANGE~


• 
  File Name: EFIubuntugrubx64.efi

Part C
Choose boot order. Put your preferred boot as the first option.