In Linux, master boot record (or MBR) is a reserved space in disk storage which stores an OS bootloader as well as disk partition table. If GRUB bootloader is not loaded properly, you can try re-installing GRUB bootloader on MBR. Here is how to install GRUB bootloader on MBR on Ubuntu or other Ubuntu-derivatives such as Linux Mint.
It's assumed that /dev/sda is the disk which contains a boot partition.
First, using fdisk command, verify that the boot partition (typically the first partition) in /dev/sda is tagged as "bootable". If the partition is bootable, it should show "*" under "Boot" column in the fdisk output.
If the boot partition is not bootable, mark it as "bootable". You can press "a" on fdisk to toggle a bootable flag. After changing a bootable flag, make sure to press "w" to write the change to disk.
After this, use the following commands to install GRUB bootloader.
$ sudo update-grub
Finally, reboot and check if it can boot from disk.
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