Knowing Ubuntu, desktop users have already been alerted about this update, that must be installed. But for Ubuntu Server administers, it must be manually performed.
What you need to do is:
1) Update your repositories:
$ sudo apt-get update
2) Verify the kernel installed version
$ uname -r2.6.32-25-generic
My kernel version is generic.
Install the right kernel version and dependencies:
$ sudo apt-get install linux-generic
And reinstall the server.