How to restore user account from different boot?
Boot to your Recovery HD or the Yosemite volume. Then open from the menubar Utilities -> Terminal (Recovery HD) or Go To -> Utilities -> Terminal (Yosemite)


Enter cd /Volumes/name_of_your_Mavericks_volume/private/var/db.  
List the content of the folder with ls -laO. A file with the name .AppleSetupDone should be listed amongst other files and folders.
Then enter rm .AppleSetupDone booted to Recovery HD (or sudo rm .AppleSetupDone booted to Yosemite) and then exit. 


After rebooting to your Mavericks volume you will be asked to configure your OS X. Create a new temporary admin user. Log-in with the temporary admin user and repair your main admin user with Directory Utility.