I think that this problem usually crops up when KDE is trying to restore settings from the last session, but for some reason can't. Sometimes when I kill the desktop (ctrl+alt+backsapce) and try to log back into KDE I get stuck at 85% percent. Usually all I need to do is reboot to solve the problem.
If the problem persists, try renaming your /home/user/.kde directory to ".kde.old" or something. You can do this via command line or by logging into GNOME or another desktop. When you log back into KDE, it'll reset all your settings and create a new .kde directory. Whatever problem it was getting stuck on at 85% should have gone as a result.
Of course, all of your user settings (colours, themes, font settings, bookmarks, menu setup) will have reset to the default. If you like, you can try copying over some of these settings from .kde.old, just make sure you don't copy over whatever folder/directory was corrupted in the first place and caused KDE to stop booting (this might be a case of trial and error.)
Hope that helps,