Macbook Pro screen goes black after disconnect from external display

Problem: After disconnecting from the external display, my Macbook Pro's screen went black. Programs were still running, but the screen was unresponsive. Had to force restart it.

Details:

Okay, so I recently got my Macbook Pro with Retina display. Since my old desktop computer is pretty much kaput, my Macbook is my main computer. However, I have a big, new-ish LCD computer screen that's perfectly good. I wanted to heckle watch a movie with a friend that's across the country, so I turned on Skype, hooked up my external screen via an HDMI cable, and let it run.

Everything worked great until it came time to disconnect. Ejected the DVD, unplugged the USB CD/DVD drive. I went to the little monitor icon in the menu bar, and selected the retina screen. Then I disconnected the external monitor-- and the screen on my macbook just went black. Skype was still working, but the monitor remained dark. Wouldn't respond to trackpad or keyboard. Tried to plug the external monitor back in, but that did nothing. It wouldn't connect again. I had to force restart the computer to get it to work normally.

My question is; what happened? Did I do something wrong? It doesn't make sense that my screen should just go black like that.

I'd like to be able to use my external monitor again for this purpose, but I can't if it does this. I've never done this before, so I know I might have goofed up on something. Any insight into what's going on would be greatly appreciated, thanks.

Macbook was also plugged into power, gigabit ethernet / Thunderbolt adaptor + ethernet cable, external USB CD/DVD drive and my USB headset.