[GUIDE] How to revert FileVault 2 if you are not able to enter your password: for example, USB keyboard is not working.
1. Boot into Recovery HD (where you can restore macOS, have Disk Utility)
2. Select Terminal (top menu)
3. diskutil corestorage list
4. Look for Logical Volume XXXXXXXX-XXXX-XXXX-XXXXXXXXXXXXXXXX and copy XXX...XXXX (NOT from Logical Volume Family!)
5. diskutil corestorage unlockVolume UUID (paste copied UUID from step 4)
6. diskutil corestorage revert UUID (paste copied UUID from step 4)
7.1) If it's your first boot after you've enabled FileVault 2, then type reboot in Terminal and boot macOS as usual.
7.2) If it's NOT the first boot after you've enabled FileVault 2, then diskutil corestorage list and check conversion status, type reboot ONLY when diskutil corestorage list shows you NO encrypted volumes!