This happened to me once, about a month ago, but since my Volt advisor hadn't heard of this before I'm posting it here.
1. Came home after a three day trip in Mountain Mode. I switched out of MM a few miles from home, so there were a few miles left on the battery.
2. Opened the charger door but didn't plug in right away. (PS we have 3 Volts at work sharing 1 charger and we do that all the time.)
3. About an hour later, I connected my 120V charger. Nothing happened. The car didn't beep, didn't light up yellow, literally nothing. The charger was normal with all its green lights on.
Here's what I tried: unplug and replug. Unplug the charger from the wall outlet and replug. These had no effect.
Here's what worked: close the charger door, then open it again with the remote fob. Now when I plugged in, the light immediately went yellow and then green.
Is it possible that the charge port is ignored if the car thinks the charge door is closed, and that it thought the door was closed in this case?