Why does OS X show a forgot password prompt when trying to boot or wakeup screen? Something came up today that bugged me a bit, I left my laptop on battery power before going to bed. When starting it up the battery was dead. I plugged it in, and here is what happened:

The startup progress bar came up for a while and then a forgot password prompt came up, with questions like:
"set a new password"
+ 2 other options that I can't remember (they weren't relevant)
and a previous and next arrows.

Puzzled, I hit previous and the dialog froze. I proceeded to restart again and ended up on the same dialog, I reset my password to the same as my previous and that was it.

Any idea what might have happened?
Brian's answer was right on. The issue in my case was related to the backlight. Here's how I got around it:


While the password dialog is up, move your mouse cursor to the top of the screen until you get an apple menu.
Select "Restart" from the Apple menu
Use a flashlight to shine a light through the Apple on the back of your MacBook screen and you should see that the computer is on, but the backlight is not. 
Use the right and left arrow keys to toggle between accounts and hit enter to login on the one you want, using the flashlight to see what you are doing.
Once logged in, the backlight should function normally.


I have not determined the root cause yet.