I have Lubuntu 11.10 installed on /dev/sda3, and Xubuntu 11.10 on /dev/sda5. A while ago, while being on Lubuntu, I made a mistake somewhere in creating a Live USB: by mistake, I installed a Live USB bootloader into /dev/sda3. This didn't result in any problem at that time.

Today, I updated the kernel. I had to restart Lubuntu. In Grub, Lubuntu suddenly didn't appear anymore, and I booted automatically in Xubuntu. I tried to run update-grub and tried to use grub-customizer to get Lubuntu back in Grub, but this didn't work. I ran os-prober, but it doesn't show me Lubuntu.

Then, I tried to add a new entry to /etc/grub.d/ on /dev/sda5 called 12_lubuntu. It contained the following:

#!/bin/sh -e
echo "Lubuntu"
cat << EOF
menuentry "Lubuntu" {
set root=(hd0,3)
linux /boot/vmlinuz
initrd /boot/initrd.img
}
EOF

After doing that, I ran update-grub and with grub-customizer, I wrote the Grub-configuration to MBR, that is: /dev/sda.

Suddenly, Lubuntu appeared in Grub. I tried to launch it, but when doing this, the following message appeared:

Error: You need to load the kernel first

How can I make Grub start Lubuntu again?