If you are using dual boot (GRUB) , NVidia drivers, and answered yes to keep your existing version of menu.lst, then you might have the same problem as me.
After I upgraded to Ubuntu 9.04, I get the following error message when restarting the machine:
Ubuntu is running in low-graphics modeAlthough I have restricted drivers enabled, I tried to activate the NVidia driver on System/Administration/Hardware Drivers and nothing happens.
The following error was encountered. You may need
to update your configuration to solve this.
(EE) NVIDIA(0): Failed to load the NVIDIA kernel module!
(EE) NVIDIA(0): *** Aborting ***
(EE) Screen(s) found, but none have a usable configuration.

If I open a terminal window and run the following commands below, and then try to reactivate again, you will be able to see some error message displayed on the terminal window:
sudo killall jockey-backendIn my case, I got the following error message:
sudo /usr/share/jockey/jockey-backend --debug -l /tmp/jockey.log
FATAL: Module nvidia not found.The NVidia module cannot be found because I am running an old version of the Kernel (remember that I said to keep my existing menu.lst version!!) and the NVidia driver is compiled to the latest version.
To check the kernel version I am running I just use the command "uname -r", and it returns 2.6.27-11-generic, but Ubuntu 9.07 comes with 2.6.28-11. So, I need to manually update my menu.lst file to be able to boot to the latest kernel version.
Warning: Only update your menu.lst file if you have done this before. This is not recommended for users that are not experienced with changing menu.lst file. If you are not an experienced Linux user, it is better to not proceed with these changes.
First, run the following commands to see that you have vmlinuz-2.6.28-11-generic:
ls /boot/*2.6.28*You should be able to see vmlinuz-2.6.28-11-generic and initrd.img-2.6.28-11-generic. Then, backup and edit your menu.lst file:
sudo cp /boot/grub/menu.lst /boot/grub/menu.lst.bakThe safest way is to duplicate your first boot menu entry, then change only this new entry to use the latest kernel version. In my case, I changed it from 2.6.27-11 to 2.6.28-11. I also changed the title to 9.04. This way you still have your previous entries in case you have any problems rebooting and need to restore your previous menu.lst from the backup copy (menu.lst.bak).
sudo gedit /boot/grub/menu.lst
Next time you reboot your machine, you can select the first boot entry and then you will see the following:
* Running DKMS auto installation service for kernelAnd then your Ubuntu will be loaded with the proper video resolution!!
* nvidia (173.14.16)...
If you have any problems rebooting your machine with the new entry, you can reboot it using an existing entry (or Ubuntu CD) and revert the changes you made by restoring the backup copy: menu.lst.bak (manually created) or menu.lst~ (created by gedit).