I do not know if this will work. Log into the Ubuntu desktop and open a terminal and run these two commands.
That is if you only have one hard disk in your machine (sda) and Grub is installed in the MBR of sda.
sudo grub-install /dev/sda
The first command updates the Grub configuration files and as you are doing this from the Ubuntu desktop it may overwrite the instructions put there by the Kubuntu desktop. The second command writes Grub into the MBR of sda. We sometimes need to do this to get changes to Grub configuration to take effect. This will not remove the Kubuntu desktop.