Help! GRUB is broken after removing Kubuntu

It seems like your GRUB boot information got deleted when you removed Kubuntu. My recommendation is to boot from a live USB, mount your drive, and back up any important data. If you're feeling adventurous, you can try loading the .img files in emergency mode, but that can be a hassle if you haven't done it before.

In cases like this, it's usually Windows that messes with GRUB when they're sharing a space. However, since you uninstalled Kubuntu, your GRUB probably got messed up there. I'd recommend booting from a live session, using the boot-repair tool to fix GRUB for Ubuntu. Don't forget to set Ubuntu as the first boot option in your BIOS afterwards.

You really just need to fix GRUB. You can check out this guide on the Ubuntu help site about installing GRUB. You'll want to boot into your existing install through the GRUB rescue, use a boot repair disk, or chroot in. SuperGrub2 might also help you reinstall GRUB if you can boot from it.

When you remove an OS like Kubuntu, it often messes with GRUB because the other installations might use it to boot. The simplest fix is to boot from a System Rescue or Super Grub2 Disk to reinstall GRUB. Just a heads up, System Rescue has some issues booting from Ventoy, so it's best to write it directly to a USB stick. Good luck getting it all sorted!

One lazy solution I've found is to just install another Linux distro in the empty space you created. It can set up a new GRUB that recognizes all your other operating systems automatically!