Broken MacBook boots grub
If you have multiple boots on your computer have you tried holding options when your computer is restarting? I use this to boot into different OSes.

Otherwise, you should be able to install MacOS by using internet recovery. I have replaced an SSD with no partitions on it and internet recovery helped install the OS fresh on the new disk. Here is an article that helps you get into internet recovery. TL;DR hold command-options-r while your computer is booting up.