Deleted Boot MBR UBUNTU 18.04/ how to repair

use a live CD and install boot repair:

 

Add The PPA

It might seem counter-intuitive, but you can enable a PPA and install software on a live CD. The utility that you need is available through a PPA, so this is the procedure that you'll need to follow in order to use it. Start by enabling the PPA and updating Apt.

$ sudo add-apt-repository ppa:yannubuntu/boot-repair
$ sudo apt update

Confirm that you want to install the PPA when asked.

Install Boot-Repair

Now, you can install Boot-Repair like you normally would with Apt.

$ sudo apt install boot-repair

 Finally, you can run the utility. You can use the same terminal window that you're working in to launch it directly.