I did three upgrades and a couple of fresh installations without a hitch.
This appears to be the norm. However, a few people have had your experience,
unfortunately. Does it list recovery as one of the options? Try that.

If not, then I would download the Live CD and burn it. Then boot into it and
back up your data. Then I would do a fresh installation.

If this is not to your liking, you could try to rewrite grub, also from the
Live CD and see if the problem is superficial or runs deeper. If it is
deeper then you can try to watch the boot process by getting out of
graphical mode and watch to see where it fails. Any Live CD could be used to
re-write grub.

I am not sure which grub you have installed. It sounds like grub2, but an
upgrade should keep legacy grub and not try to translate it. If it did try
that then that may be the source of your problem.