I reinstalled Ubuntu (it was working right) and now the Grub menu is not appearing. I've used the Boot Repair tool too, it just shows a black screen and the underline flashing.
Have you tried it with command in terminal? Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command(s) below:
if this does not help, then try
and finally at end
this should work.