Ubuntu 11.04 has many known issues posted on its release notes, one line said:

On certain systems, a bad interaction between Unity, the GL driver and the kernel may cause the system to hang. (740126)

This is a frequent screen frozen issue, and it’s said has something to do with the kernel. Default kernel come with Ubuntu 11.04 is 2.6.38-8, and here’s how to upgrade to Linux kernel 2.6.39-0.

Update: Kernel-ppa currently is down, there’s no kernel packages. Before adding the ppa check ppa:kernel-ppa/ppa. As an alternative, you can download latest kernel packages from http://kernel.ubuntu.com/~kernel-ppa/mainline/

Press Ctrl+Alt+T to start a terminal window, and do following steps:

1.) Add the kernel ppa and update your system:

sudo add-apt-repository ppa:kernel-ppa/ppa

sudo apt-get update

2.) Check available kernels with the command:

apt-cache showpkg linux-headers

kernel should be in list.

3.) Run the command to install kernel

sudo apt-get install linux-headers-2.6.39-0 linux-headers-2.6.39-0-generic linux-image-2.6.39-0-generic --fix-missing

Finally, reboot and check kernel version under System Monitor -> System tab.