I have installed Ubuntu 16.04 on my laptop. A few days ago I have installed Ubuntu 18.04 on my external Flash drive. After testing new Ubuntu and removing flash drive from computer, I decided to boot again - and I got grub rescue menu.
Why my previous grub was deleted ?
grubmost likely was not deleted; it'll still be there in /boot/grub/ (stages 1.5-2), but stage 1 is the MBR and the 18.04 install would have asked you if you wanted grub installed; your MBR was then pointed to the place 18.04 was installed (you have only one MBR). The following may help you to restore it https://askubuntu.com/questions/88384/how-can-i-repair-grub-how-to-get-ubuntu-back-after-installing-windows (i realize it's not from a windoze install; but same effect) – guiverc May 10 '18 at 07:52