11/3/2017 update: It's happened again, this time it wasn't encryption on my boot drive but rather my Time Machine Hard drive. After PRAM reset it showed up. Fortunately I was able to login using my Time Machine's password, not my admin password. I did this after trying (unsuccessfully) to boot Time Machine. When I typed in my Time Machine password, and booted off the Time Machine drive. Once done booting, I set my boot drive to my normal drive in my recovery menu. I was able to boot again, problem free. The problem went away this time without all the steps below.
I recently updated to OS X 10.11 El Capitan and received an “Update Needed” login screen, that looked like the following, but without a guest user.
After a bit of digging, it was apparent that FileVault was the culprit (encryption). I booted up my copy of Mountain Lion and was able to access my drive with my encrypted password. However, it was apparent I needed a newer OS to decrypt it as the option was greyed out.
I pulled my drive and stuck it in an external enclosure and fired up my MacBook Pro.
1) Access the drive from another copy of OS X
Boot your problematic Mac in Target disk mode or plug the drive into an external enclosure or simply boot another copy of your OS or recovery partition, the requirement is Yosemite or El Capitan (and perhaps Mavericks) to decrypt Yosemite/El Capitan drives,
2) Launch Disk Utility
If you’ve booted from your recovery partition, disk utility is available on the menu> Otherwise, Disk Utility can be found in its normal location: /Applications/Utilities