From time to time, the product's binary preference files (tfprefs.rsc and stfprefs.rsc) become corrupted which can prevent the product from running properly or even loading. The solution is to rename or delete them to restore the out-of-the-box or default preferences.
on a windows XP machine, the preference files can be found in:
c:\documents and settings\<firstname.lastname>\localsettings\Bentley\MicroStation\8.11\<some generated number>prefs\
on a windows 7 machine, the preference files can be found in:
C:\Users\<firstname.lastname>\AppData\Local\Bentley\MicroStation\8.11\<some generated number>\prefs
The structural preferences are stored in a binary resource file, stfprefs.rsc. When the software is not running, you can try backing up this file by renaming it to something like stfprefs_rsc.001. If you do this and restart the product, you will start with the default "out-of-the-box" preferences. See if that makes a difference.
If that doesn't work, there are a couple of other preference files such as structural.upf (or whatever user name you're using) and tfprefs.rsc in the same folder and you can try the same thing with those.
How to determine where the preference files are stored