Error: You need to load the kernel first in Grub

Boot-Repair

The majority of grub boot issues can be resolved through using boot-repair - as this community wiki explains, you can install this on a liveCD/USB or directly in a working ubuntu install and it will attempt to find all O/S installs and repair the overall Grub MBR.

Manual Grub2 Installation

If boot-repair fails to work its magic then the traditional manual grub2 installation method described in this community wiki should be investigated.

Search on that web-page for the section Copy LiveCD Files. Replace the example /dev/sdxy statements with the partition that you have installed lubuntu in - for example /dev/sda3

If you are unsure which partition you have installed then you can run the Boot-info-script found on SourceForge.

In the results look for partitions types of ext4 or ext3 - these are the common Ubuntu filesystems unless you previously elected to use something like btrfs or reiserfs etc.

Kernel reinstallation

If the issue is a corrupt kernel installation then you can investigate reinstalling the kernel from a live CD. This is described in this excellent AskUbuntu Q&A

By installing the kernel correctly, Grub should be able to see the Lubuntu installation correctly.