What I can guess is that you might to remove the old kernels. You can do this by
Note down the current version of kernel
dpkg -l 'linux-image-*' | grep '^ii'
Note down the kernel version(s) (the numbers after the linux-image, ie x.y.z.-generic)
(Include "-generic too")
Then run this command to delete old kernels WARNING : DO NOT DELETE THE KERNEL WHOSE VERSION YOU PREVIOUSLY NOTED DOWN IN "uname -r"
sudo apt-get purge linux-image-x.y.z-generic
(In my case it will be
sudo apt-get purge linux-image-220.127.116.11 since linux-image-18.104.22.168 is the version I noted down in "uname -r")
Lastly update grub with the commands
(Running both wont do any harm to the computer)
Hope this helped you.