@grahammechanical provided a link to a thread at the Ubuntu Forums, so first I thank him for that.
Are you having these problems?
- "Cannot display this grahics mode"
- GRUB_GXFMODE - auto results in blank screen problems on Startup? (this includes purple or black screen, flashing cursor, stuck at splash screen...)
- No grub menu?
The post contains a pretty exhaustive list of keyboard shortcuts to help diagnose graphics problems on Linux systems, e.g.
CtrlAltF1
Switch to the first text terminal. Under Linux you can have several (6 in standard setup) terminals opened at the same time. Terminals start as tty0 and go up from there. Most of the time the normal boot text console, that is present "under" the GUI or Xsession (in Ubuntu) is tty1, so you would press to get to it...
CtrlAltF7
Switch to the first GUI terminal (if X-windows is running on this terminal).
There's also a short troubleshooting flow chart:
Do you have a grub menu?
- Yes: Go to step 2.
- No: While booting, Press shift key multiple times (don't hold it down) to see if the Grub menu will come up
- If you get the menu now, go to step 2
- If still no menu, comment out
GRUB_HIDDEN_TIMEOUT=00
in /etc/default/grub/ and rerun grub-update from a LiveCD.
- If you get the menu now, go to step 2
- If still no menu, reinstall grub and start over from step 1, because it seems that grub is not booting.
Does the Linux kernel boot?
At Grub Menu, go into edit mode and boot into a text console
- Yes it boots: Go to step 3
- No it fails to boot: Messages will be verbose on what is loading, what are warnings and what are error messages. Shortcut keys will start to work as the kernel modules load. If it stops at an error, you will be able to use the shortcut navigation cuts to review the errors. If it is a kernel error, you may be able to reinstall or renew the kernel image. If it is a device module problem, at least you have somewhere to go to reload that device module or driver. The goal is to get a "booting kernel."
From the Grub Menu, try to boot in Rescue mode/low graphics.
- Yes it boots: look for additional drivers and install recommended driver.
- No it fails to boot: go to step 4 to verify that the Linux kernel will boot.
Can you boot a graphical X session from a text console session?
From the command line type:
sudo service gdm start
- Yes I get a graphical X session: No black screen, no problem; it should boot straight from the grub menu.
- No it fails to load: Reboot and start testing and changing gfx_modes and kernel boot graphical modes, still booting into the text console before you try to start an X session. Going this way, you will have more of a chance to be able to toggle between a graphical session or text terminal session (sometimes). At a text console, at least you have the ability to install files and make changes to config files, and if you can get back into a command prompt, you could then stop the gdm service that is locked.
You can stop the gdm service via
sudo service gdm stop
There's even a table of contents post that can help find the post in the thread that might help your particular situation.
Again, this all comes from a thread at the Ubuntu Forums.