In general if you need to downgrade, you are looking at a reinstall; upgrades in Ubuntu are one-way trips.
Because of the amount of churn in the X-window / kernel situation currently, I like to test with live CD's to very all my hardware still works *before* allowing upgrades. A suggestion for next time.
Does the text-mode login work? E.g. if you press CTRL-ALT-F1 then Enter, do you get a prompt "login:"? If so, you might want to try:
That might or might not improve things any.
apt-get upgrade --fix-broken
Another possibility is to try moving to 10.04 "lucid lynx"; that has another 6 months of video development behind it.