I applied the free upgrade to Windows 10 on my computer. After the installer got to around 90% and made a rebooting, the BIOS screen appeared with Missing operating system error. That prevented me from booting back in Windows and actually turn the device to be unusable completely.
A number of users get the Missing operating system or operating System Not Found error during the upgrading process of Windows 10. That may be caused by those reasons , such as corrupted Hard disk,hard disk is not detected by BIOS, or your system partition is not active. You could try the method to fix the issues.
Note: This process is very safe, so any type of data through this process won’t be deleted.
* First you need to find your bootable cd/DVD or create Bootable USB stick.
You can download the Windows USB/DVD Download Tool and run it to make one copy of the ISO file on USB flash drive, or other media by following instruction here.
* Power up your PC and press F2 or delete button from your keyboard to enter the BIOS setting, and set it up to boot from CD or USB.
* Attach the Bootable USB or DVD in your tablet or computer, and make a re-booting.
* Under the Install Now button., you should see the Repair Your Computer. Click on it. You might select your OS from list if needed.
* Open CMD (Command Prompt) in Repair Your Computer screen.
* Type those command and press Enter to execute it.
* Then repeat the same way for running this command:
* Finally remove your cd or USB stick and reboot to check if it is fine
If it is still not working, open the Command Prompt again by the above method,
diskpart to execute it, then type
list volume to display all volumes.
* Find your system volume number where the Windows is installed.
* Input the command and press Enter:
select volume ***
For example, select volume 3
ACTIVE to active your system partition.
* Then input exit to close cmd and restart the device.