For reinstalling GRUB bootloader, there is my favorite article from HTG here https://www.howtogeek.com/114884/how-to-repair-grub2-when-ubuntu-wont-boot. Simply follow the command line section there so you do not need to install anything. I always give this to everybody asking about their bootloader needs to be repaired.
The one lcerf pointed out above is great (thank you!) but the HTG's is far more simpler.
