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.