This did not happen in GNOME 2 / Ubuntu 10.04 Natty / Linux Mint 11, and happens only after I upgraded to GNOME 3 / GNOME Shell / Ubuntu 11.10 Oneiric / Linux Mint 12.
To solve this problem when the screen is blank:
- Press Ctrl+Alt+F1 (to go to the console).
- Close the laptop lid.
- Reopen the laptop lid.
- You should be able to see the Linux console. Enter your username and password to login.
- Type:export DISPLAY=:0
xset dpms force on
( Note : if you type "xset q" right after the "export" command, you'll notice that "Monitor is Off", which is both strange and scary )
- Press Ctrl+F7 to go back to your desktop.
You may also want to configure your Screen Settings to disable turning off the screen.
I have not yet found a more permanent, properly working solution. So right now the above is a temporary hack that works.
You can learn more about Ubuntu and Linux Mint in Ubuntu Unleashed 2011 Edition: Covering 10.10 and 11.04 (6th Edition) !