Ytdyklly

I installed Ubuntu on a Dell XPS 13 laptop that had Windows 10 preinstalled on it. I installed it from a USB flash drive on a partition.
When I boot my computer I can only boot into Windows 10 and Ubuntu is nowhere to be seen.

If I boot into my USB then I can see that Ubuntu is installed, but I can't get to it from the BIOS boot menu.

Disable the hibernation mode and Fast Boot in Windows.

Open command prompt as administrator and execute :

powercfg /h off  

Open the legacy version of the Windows Control Panel (not the modern version).

Select Energy Settings, enable show hidden settings and uncheck Fast Boot.

After having done this - shutdown the computer completely - do NOT reboot.

Boot into the BIOS and select Ubuntu as the default operating system to boot.

Update :

In case there is no Ubuntu entry to be found in the BIOS / UEFI settings,

re-install the GRUB boot loader to your Ubuntu installation in EFI mode.

Boot from the Ubuntu installation media - open a terminal and execute:

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  

Note:

sdX = disk | sdXX = efi partition | sdXXX = system partition

To identify the partition numbers use GParted - it is included in the Ubuntu installation media.

Boot into BIOS and select Ubuntu in UEFI settings to be the default operating system to boot.

I had the exact same problem. What fixed it for me was this:
I have 2 different hard drives installed. It took me forever to figure this out but was solved it was changing the boot priority from the first to the second drive. Probably the boot manager for ubuntu was installed on the other drive so it couldn't find it and booted straight into windows.

Shutdown. Then press F12 on boot and use arrows to choose ubuntu (note lowercase).

Also, you should try shutting down, press F2 to open BIOS/UEFI settings and disable Secure Boot and make ubuntu top priority. Then GRUB can boot Windows.

Immediately after the BIOS/UEFI splash screen during boot, with BIOS, quickly press and hold the Shift key, which will bring up a GNU GRUB menu screen. With UEFI press (perhaps several times) the Esc key to get to the GNU GRUB menu screen. Select Ubuntu from the GRUB menu and press Enter.

From Ubuntu open the terminal and type:

sudo update-grub && sudo reboot

If this doesn't work try repairing the GRUB bootloader with Boot Repair. Open the terminal and type:

sudo add-apt-repository ppa:yannubuntu/boot-repair  

sudo apt update  

sudo apt install -y boot-repair

sudo boot-repair  

Open the Boot Repair application and select Advanced Options -> Main Options tab -> check Reinstall GRUB and Unhide boot menu as shown in the below screenshot. Click the Apply button. Then reboot the computer with sudo reboot

If the OSs were installed in different modes, dual booting Windows and Ubuntu can't work. If your Windows is installed in BIOS mode, it is recommended to install your Ubuntu in BIOS mode, but if it's installed in UEFI mode, then do the same with Ubuntu. To check if your Windows is installed in UEFI, press the keyboard combination Windows + r then enter the command msinfo32 in the Run window. In the new window that opens up look for the entry after where it says BIOS Mode.

          ^{BIOS vs. UEFI}

If you have installed Ubuntu in legacy mode on the same drive with GPT partitioning, you can use Boot Repair's Advanced options to uninstall grub-pc and install grub-efi-amd64. That converts the Ubuntu installation from BIOS boot to UEFI boot, the same firmware as most recently manufactured laptops with Windows pre-installed have.

Converting Ubuntu into UEFI mode

1. Start Boot-Repair, and select Advanced options -> GRUB location tab.


2. If you do not see a Separate /boot/efi partition option, this means that your PC does not have any UEFI partition.


3. 
   

   If you see a Separate /boot/efi partition option, put a checkmark in the checkbox to the left of it, then click the Apply button in the lower right corner.

   
   
   

   

   
   



4. Set up your BIOS so that it boots the hard drive in UEFI mode. The way to adjust this setting depends on the specific model of the computer, but generally this setting is located in the boot priority settings under the Boot tab of the BIOS/UEFI setup utility.

For more information about converting Ubuntu into UEFI mode review https://help.ubuntu.com/community/UEFI in the section about Converting Ubuntu into UEFI mode.

The grub bootloader can also be converted in the opposite direction from UEFI to BIOS. Linux can boot fine from a GPT disk in BIOS mode. See this answer: Convert from EFI to BIOS boot mode