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?

Solution 1:

Brian's answer was right on. The issue in my case was related to the backlight. Here's how I got around it:

1. While the password dialog is up, move your mouse cursor to the top of the screen until you get an apple menu.
2. Select "Restart" from the Apple menu
3. 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.
4. 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.
5. Once logged in, the backlight should function normally.

I have not determined the root cause yet.

Solution 2:

Although I didn't run out of battery power, I had the same issue this morning on my 15-inch, Mid 2012 MacBook Pro running Yosemite final. This was not the same laptop I used for the beta which was upgraded to final yesterday and has not yet failed.

Discovered the problem. The system was prompting for my FileVault password but with the screen backlight off. It appears that by pressing the enter key repeatedly in an attempt to get the system to respond the startup password reset mode was launched.

I ran into the issue last night at 95% battery life remaining when I performed a hard power reset while logged in after the lock screen became unresponsive. I thought I was seeing persistence on the LCD under the lighting conditions I was working with last night and experimentally tried pressing right arrow to select the "Guest" account.

This is similar to MacBook display backlight doesn't turn on after waking from sleep with the exception that logging in does reactivate the backlight/display.

Resetting the PRAM and SMC did not resolve the issue and as I did not experience the problem prior to upgrading to Yosemite I believe it's a software, rather than a hardware issue. For the moment I can only recommend adjusting the angle of the display under lighting to see if the password prompt is displayed and logging in to unlock FileVault.

UPDATE: I've gotten my hands on an Early 2013 15" Retina Macbook Pro and performed a full restore from backup. I haven't seen the issue on this laptop, nor have I seen the issue on my Late 2010 11" Macbook Air. This appears to be affecting only specific hardware.