There are many reasons and solutions for this.
1) Removing quiet splash
When you are on the grub menu (the menu which you select what OS to boot), make sure that your selection is on "Ubuntu" and press E. You'll see a line like this one:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
And remove "quiet splash". To save grub like this, press F10.
2) nomodeset
If the first one doesn't work, after deleting "quiet splash", add "nomodeset" in their place. If this works, it also means that you probably have nvidia GPU or Dual GPU.