When you upgrade to High Sierra, not only are you getting a new OS, but if your Mac has all-SSD storage, you are getting an all-new filesystem: APFS. This means your startup disk gets converted from HFS+ to APFS during the High Sierra upgrade.
More changes means more chances for things to go wrong. A colleague walked into my office today: he’d just upgraded his Mac to High Sierra, and now it was sitting at the Filevault pre-boot authentication screen. But instead of an icon for his account, there was a hard drive icon and the label “Disk Password”. His account password did not work to unlock this disk, and neither did the Personal Recovery Key.
Looked like the APFS conversion didn’t do everything it needed…
Fortunately Nick McSpadden had seen this before and let me in on a fix:
- Boot into Recovery HD
- Open Terminal.app
- Run
diskutil apfs listto get the ‘regular’ boot drive (disk2s1 in this case) - Run
diskutil apfs unlockvolume disk2s1to unlock the drive. The user’s normal password worked here. - Run
diskutil apfs updatePreboot disk2s1
This last command sprayed a ton of text to the screen. After this, we rebooted, and the expected disk unlock screen came up, with an icon for the user’s account. He was able to successfully authenticate and the Mac proceeded to boot. Crisis adverted!
Managing OS X 
Thanks for sharing the fix.