I've upgraded Ubuntu 10.10 to Ubuntu 11.04 on my HP laptop, and the new Ubuntu started freezing after booting. It didn't boot in the recovery mode either. I tried to install Ubuntu 11.10, but it didn't boot as well.
Tried to use the nomodeset option (added it by pressing the E button in the grub), but it didn't help either. I don't really know what causes the problem. Hitting the ESC button or CTRL+ALT+F1 before freezing didn't help to show anything on the screen.
Then I decided to install Kubuntu 11.04. It booted without any problem. I didn't like Kubuntu, so I installed unity-2d and lubuntu-desktop from it by running
Code:
sudo apt-get install unity-2d lubuntu-desktop
in the terminal. I chose the GDM windows manager because I got used to it.
Now I can choose between unity-2d, lubuntu (LXDE) and kubuntu when logging in.
I guess the problem is caused by unity. But instead of installing Kubuntu I would recommend to install Lubuntu (doesn't have official endorsement from Canonical) or Xubuntu. Both of them use lightweight windows managers.