After updating GRUB in Ubuntu I got this error and I was so afraid of this. But a YOUTUBE video helped me to recover from this. May this post can help you too.
I was using SAMSUNG laptop. Here is what we need to do.
Go to BIOS->BOOT
You will see an option ‘SECURE BOOT’ and you need to disable it. Also when you disable it you will see an other option appears named as ‘OS MODE SELECTION’. If you disable ‘SECURE BOOT’ and your OS still cant boot then you need to change the options given in ‘OS MODE SELECTION’.