I accidentally removed the BIOS boot option for Linux, but fortunately, I found this very detailed guide on how to restore the GRUB bootloader for Linux.

I believe this will work in most cases, even after installing the Windows bootloader, which usually overwrites Linux boot settings.

Below are some notes after I did this (more details in the link above):

1. Create an Ubuntu boot disk. The download is usually fast, but the slow part is writing to the boot USB (which took 20 minutes for me). See how to create a boot disk (on Windows) here.

2. Find the Linux partition:
   
   sudo fdisk -l
sudo blkid

3. Check for EFI boot:
   
   sudo fdisk -l (look for a partition with the type 'EFI System').

4. Mount your partition:
   
   sudo mount /dev/sdXY /mnt
   
   This is needed to chroot into your Linux mount later.

5. Bind mount necessary directories:
   
   for i in /sys /proc /run /dev; do sudo mount --rbind "$i" "/mnt$i"; done

6. If you have EFI boot:
   
   sudo mount /dev/sdXY /mnt/boot/efi

7. Chroot:
   
   sudo chroot /mnt

8. Update GRUB:
   
   update-grub
   
   After this step, you may restart if it's fixed. Otherwise, continue to step 9.

9. Reinstall GRUB if not fixed:
   
   grub-install /dev/sda
update-grub (to detect and add Windows to the GRUB menu).

10. Check if the EFI UUID is correct, and update it if needed:
    
    blkid | grep -i efi
grep -i efi /etc/fstab

11. Finish:
    
    exit
sudo reboot

There are many steps, but the detailed instructions make them easy to follow. It's easy to Google any terms you don't understand, so don't worry.

I hope this helps fix your Linux system!