The first thing to do is open the CUI, and the easiest way to do that is to type CUI (and then of course hitting the Enter key) on the command line. In the upper-left panel, you'll see the word ACAD. Click on that, and right-click. You'll see a choice in the menu to Reset ACAD.cui. Pick that, and everything should reset itself to the way it's supposed to be.
If there's something terribly wrong, you may need to do a repair of your software. This is also quite easy. From your Start menu, find the Control Panel, and once you're there, select "Add and Delete Software"
You're not going to Uninstall at this point, so don't worry!
Find your Autodesk application in the list, once it eventually finishes populating itself.
Select the "Change/Remove" button. (I know it feels like you're going to lose your program, but you won't accidently delete it, so stay with me here.) That button brings up the menu pictured here, the Installation Wizard. Choose the Repair or Reinstall button, and it will repair most program errors - just like that!
As long as we're on the subject, I wanted to mention something about the .cui file, and avoiding possible problems. There are two things I always do before I start modifying any toolbars or workspaces or anything else. First I make a copy of the acad.cui file (and the same would go for any other Autodesk product, only the name would change, depending on what you have. Like you didn't know that! I apologize for stating the painfully obvious) I know there's a backup file, but I just like to save one in a completely different location.
If you don't know where your acad.cui file is stored, go to Options and pick the Files tab. In the Customization folder, you can see the path to where your program is looking for the .cui file. This is also where you would change it if needed - if the file you've been using becomes corrupt there is a file named acad.bak.cui in the same folder as the acad.cui file. You can browse and point to that file if your acad.cui gets corrupted somehow. Or you could do a "Repair" as outlined above.
The second thing I always do is save a copy of my Profile. (This doesn't exist in AutoCAD LT, so if that's your software, just skip this part.) This is done in Options, on the Profile tab. I usually make a copy of my profile right out of the box, and another after I've got things changed just the way I like them. That way, if I lose something or on the practically unheard of chance that I might mess something up, I can set my backup profile as the current one.
Both of these precautions have a drawback - you lose all the customization you did after you saved the backup copy. However, if it's a matter of your program not working at all versus not having a few favorite tools, I'd rather be set back to my original install than not be able to work at all.
And of course, there's nothing preventing you from making new backup copies of your .cui file and your Profile once you have them fixed up the way you like. Better safe than sorry, like my Grandmother would say.