Unable to login after accidentally powering down laptop as FileVault decryption of HDD was running I had turned off FileVault on my FileVault-encrypted HDD. While the HDD was decrypted, I accidentally powered down my laptop interrupting that decryption process. When I subsequently rebooted, I am able to click on my username and enter my password, but then instead of displaying the Apple logo, my laptop displays the "No entry" traffic sign and proceeds no further.

Now my OS X Lion 10.7.6 system (on MacBook pro 13" 2011) no longer boots after a restart. 

I have booted into Lion Recovery to repair permission but Disk Utility will not mount my HDD and instead reports error (-69846). I have also tried using diskutil without success.
An output from diskutil list recognizes my partition (/dev/disk0s2) as having the type FFFF-FFFFFFFFFFF-FFFF-FFFF.

How do I make my FileVault encrypted (partially decrypted?) partition mount so I can log in?
Sorry but I don't have good news for you. The "home solution" is to erase and reinstall with DU. The good news might be if you had up to date backup to restore your system from.