I think i just found the solution..
1) connect the external display (both are working), and make sure the external is the primary display.
2) use a magnet to shut the internal display off (only the external is working by now).
3) lift up the magnet and quickly close the lid.
I succeed using the clamshell mode using this method. Hope to hear good news from you all.
Macbook Air 13inch 2011
OS X 10.8.2
THANK YOU for sharing this dead simple fact !!!!!!
I had rewired all my (music) equipment, unplugging everything in the process. I use a dock (Henge Docks, highly recommendable by the way) to avoid plugging in each time I need them all the peripherals I need for my music.
This dock obviously includes the power connection. When I replugged everything, I forgot (yeah... I know) my MacBook's power supply, and didn't notice because the battery was full and everything else was working.
It made no sense that suddenly my laptop wouldn't wake up, clamshell had worked less than 12 hours earlier, I was loosing my mind.
I hope my thanks reach you. You helped me avoiding all those surely useful but very scary terminal commands (scary to the very basic user I am.)
Not trying them was a very good thing as I never use clamshell on battery, so my hilarious "problem" basically solved by plugging in what had always been plugged in...