Are you still able to get into Windows? That is, does the Windows bootloader come up first, and let you select Windows or Ubuntu? or do you immediately see the grub prompt? If the latter, then your reinstallation of Ubuntu overwrote the Windows bootloader and installed grub, but the grub installation has failed to identify the presence of any operating systems on the computer.
If the Windows bootloader is still letting you select Windows or Ubuntu, and you see the grub prompt but nothing else when you select Ubuntu, then your reinstallation of Ubuntu did not work.