Well I just had this happen, and thought I would share

Enter Recovery Mode Open the Terminal from the Utilities Menu Open the Disk Utility from the command line

/Applications/Utilities/Disk\ Utility.app/Contents/MacOS/Disk\ Utility

Select the lock drive and click mount. It will prompt for a password if encrypted

Then you can enter your root volume like normal

Enter Recovery Mode. Open Terminal app. Launch Disk Utility app from the command line by executing:

/Applications/Utilities/Disk\ Utility.app/Contents/MacOS/Disk\ Utility

Select the lock drive and click mount. It will prompt for a password if encrypted

You can then enter your root volume normally.