In most circumstances, using that Registry Cleaner's Undo or Restore feature fixes the problem, most quality products also include an exclusion option where you can add AutoCAD/LT to the list of programs not to touch.
If you use a freebie or a bargain cleaner, they don't restore or recover well. A repair reinstall of LT fixes the problem as noted HERE.
If not, you need to go a step further and completely uninstall all Autodesk products on your system as discussed HERE, then reinstalling them after a Windows restart.
Click on this for a GOOGLE search on the topic and other solutions posted.
Don't forget to backup all your LT customizations first, often, and regularly. ◦
