MBA doesn't see external display

Question

Folks,
This is *really* frustrating.  Last night I tried, for the first time, to use an external display for my MBA.  I have a Thunderbolt-to-HDMI Apple cable I bought for this purpose (the only way to do it on my MBA), and I connected it to the external display via an HDMI cable.  I plugged in the monitor, turned it on, and opened up the MBA (it was in sleep mode) and it was glorious -- full, beautiful color (well, after I adjusted the gamma and brightness/contrast appropriately to calibrate it).
So I went ahead and turned on bluetooth so I could use my BT keyboard and I was able to then close the MBA and work on my Mac just like it was a desktop.  It was the greatest thing since... well, since cheap gas.  I could see that I would be using my MBA even more than I already do.  I put the Mac to sleep and tried to see if it would revive from a mouse movement and it did, with the display working perfectly.  I put it back to sleep again and went to bed.
This morning I tried to revive it with a mouse movement and no-go.  Okay, so I opened it up -- still no life.  Hmmmm, I pressed the power button a while so it would come back on, and it finally did, but no external display.  I read online about getting it to see external displays (something I didn't have to do yesterday) and tried "Detect Display", tried turning it completely off for five minutes and powering back on, tried disconnecting and connecting the cable, turning the display power on and off... nothing.  No matter what I do, I can't get the external display to work again.
What in the #$%@# am I doing wrong?  I can't even think of anything else to try.  Leaving aside the fact that it somehow stopped working in the night during sleep mode, what can I possibly do now to get it going again?

Answer 1

I thought I'd add a couple of things here that might be helpful to others searching for a solution to this or a similar problem.
I happened to leave my MBA turned off for a while but with the external monitor connected.  When it came time to turn it back on I connected it to power (although it was pretty fully charged) because I had been reading this:
http://gizmodo.com/5938452/a-trick-to-make-using-an-external-monitor-with-your-m acbook-way-better
and in the bottom comments there was a section that read as follows:
"Closed Clamshell Mode is still a function of the OS. Here's how: close the lid of portable, plug into power, monitor, and keyboard/mouse, wake portable by clicking the mouse or hitting a key on the space bar. Important note: portable has to be plugged into power, even if the battery is full."
It was the last bit that seemed to make a connection in my mind that perhaps the big issue here was that I was trying to do things without the power connected, even with the Clamshell open.  In any case, the external monitor came back on and then worked just fine.  So I shut things completely down (power down) and restarted several times and each and every time it worked, and kept working even when I went to Closed Clamshell mode.
So I tried with the Clamshell open but NOT connected to power and I had the same issues -- no display and couldn't get it working properly again until I completely shut down, connected to power, and brought it up again.  I am now of the opinion that, at least in my particular case (with the particular thunderbold to HDMI adapter, Dell monitor, and 2011 MBA) that the power needs to be attached no matter if the battery is full or not. (Note: I realize in the above quoted paragraph the big point is that in closed clamshell mode you need to be connected to power.  Still, it seems to me it also makes a difference in open clamshell as well, at least based on my limited tests).
The MBA may still be faulty -- perhaps it should work otherwise -- but I'm happy with things the way they are now.  I will continue to play with this (I had, in the meantime, ordered both another thunderbolt to hdmi adapter as well as a thunderbolt to DVI adapter just to see if they made a difference -- they will come Wednesday and I'll give them a try to see what difference, if any, they make) but at least at the moment I feel like the key to solving this was to keep it plugged into power (it also may have helped to reset the PRAM because I had made some changes to the LCD display that may have screwed things up.  No matter what, that's still good information to know).