My GRUB menu vanished, and I cannot access it with left SHIFT. When I press the left SHIFT I see
opening GRUB and then the desktop menu.
I need it urgently because I lost my password.
When you see on the screen
If you are not still able to enter, please make sure that you are using Grub2. Earlier versions of grub (0.9x) are issued with the ESC key instead.
Boot to an Ubuntu live cd and use boot-repair to unhide the GRUB menu.
In a terminal, type:
Once boot-repair opens, click Advanced Options. Make sure that "Unhide Boot Menu" is checked and run the repairs, following any instructions you are given.
You have to do this from a live USB because you lost your password.