this sort of problem will need some more information, especially the hardware you are using and if you are using any non-standard drivers (either self installed, or via the additional hardware drivers window).
So - if you are happy to try a few things and report back by editing your question, I will likewise (if I have any further ideas), reedit my answer - obviously many of these "try this" stuff will need you to boot into your working kernel or via recovery mode.
1) Possible graphics issue? Maybe you have installed a graphics driver?
Possible solution - remove or rename your xorg.conf file
sudo mv /etc/X11/xorg.conf /etc/X11/xorg.conf.backup
2) Possible new KMS issue with your graphics card
Try booting with nomodeset as your grub option as described in this AU question and answer
3) Possible new incompatible hardware issue preventing boot
Try removing all plugged in hardware except your monitor, keyboard and mouse. Try to ensure you use a wired keyboard and wired mouse.
4) Possible the upgrade was not completed
sudo apt-get update && sudo apt-get upgrade
Any errors reported
5) Can you confirm how you upgraded your laptop - did you use update manager from v10.10 to 11.04 or via the live/alternate CD or some other mechanism?
6) Some strange power-management or interrupt issue?
Try booting with either acpi=off or noapic
7) Some sort of strange networking issue?
Try plugging in via a fixed wired connection and hardware disable any wireless card e.g. via BIOS or possibly some switch on the side of your laptop.
Please add to your question details of your hardware i.e.
N.B. also add to your question that this isnt an external screen issue as per your comment - so that that sort of information doesnt get lost with any other comments. TIA