What you are attempting to do is to upgrade from LTS to LTS which should
work, however I have never done this as I use the intervening releases. The
upgrade from 8.04 to 10.04 is a big one as there are so many changes that it
is hard to know what went wrong. Lots of potential for disaster, IMO. You
don't say what you are left with when you boot. Do you get a GUI at all? Do
you have a command prompt? Have you tried the recovery mode in grub?

Please let us know more about what happens precisely when you boot. If you
can get a GUI or command prompt then all is not lost. If it just hangs then
things are more complicated.