The boot process can't find the root partition (the part of the disk, that contains the information for starting up the system), so you have to specify its location yourself.
I think you have to look at something like this article: how-rescue-non-booting-grub-2-linux
short: in this grub rescue> command line type
ls
... to list all available devices, then you have to go through each, type something like (depends what is shown by the ls command):
ls (hd0,1)/
ls (hd0,2)/
... and so on, until you find
(hd0,1)/boot/grub OR (hd0,1)/grub
In case of efi
(hd0,1)/efi/boot/grub OR (hd0,1)/efi/grub
... now set the boot parameters accordingly, just type this with the correct numbers and after each line press return
set prefix=(hd0,1)/grub
or (if grub is in a sub-directory)
set prefix=(hd0,1)/boot/grub
Then continue with
set root=(hd0,1)
insmod linux
insmod normal
normal
Now it should boot. Start a commandline now (a terminal) and execute
sudo update-grub
... this should correct the missing information and it should boot next time.
If not, you have to go through the steps again an might have to repair or install grub again (look at this article: https://help.ubuntu.com/community/Boot-Repair)
Best Answer
On a qemu virtual machine Ctrl-Alt-Del does it (also
sendkey ctrl-alt-deletein the qemu monitor; but on my machine I can simply type Ctrl-Alt-Del -- they're not special in my "desktop environment").I'm not using virtualbox, but google says that virtualbox has a menu option (Input->Keyboard->Insert Ctrl-Alt-Del)