If you press escape in grub menu you end up at the grub command prompt.
My question is, how can I go back to the menu, without rebooting?
Type normal, hit Enter, and then tap Esc until the menu is displayed. Hitting Esc at this point won't drop you to the grub command prompt (so don't worry about hitting Esc too many times).
 
    
     
    
    Executing the normal command brought me back to the grub menu.
I found this command (and others) by pressing Tab in the grub CLI.
 
    
    Shift + Esc: What worked for me was holding down Shift and pressing Esc several times.
 
    
    