FileVault locked me out

1. Reboot your Mac and hold Command-R at the startup sound to get to Recovery Mode.
2. Open Disk Utility in the OS X Tools/Utilities dropdown
3. Select your boot drive in the sidebar.
4. Click the Unlock button, and then supply your FileVault password to unlock the drive.
5. Hold the Command key and click the unlocked volume and the drive device in the sidebar to select them both.
6. Go to the First Aid tab and click the “Repair Disk” or "First Aid" button.
7. If error repaired, run it again until it reports no errors
8. Reboot as normal

File vault is OS X built in data encryption technology , when enabled as with an unencrypted OS X volume you simply enter your account credentials to get into the system .

By going in recovery mode resetting utilities then can change an administrative password even without being logged in whether an outsider can reset your password & get to your encrypted files .

Without file vault enabled : it is possible , but if file vault is enabled these password reset routines will not work .

In a scenario if file vault is enabled & you boot in the OS X recovery HD partition & try to use the reset password utility .

In this case you will not see your boot drive listed a valid source for a system account in which to change the password .

You will have to open disk utility & unlock the volume & even then the reset password utility will only change OS X password & not the file vault password .

an apple article is there : If a flashing question mark appears when you start your Mac - Apple Support