Step 1: Reset your preferences: Close Lightroom. Hold down [Alt/Opt]+[Shift} and launch Lightroom. Click the button to delete the preferences in the dialog.
Does you key functionality come back?
It's possibly a keyboard issue, does the square bracket on your keyboard produce a [ on a blank document?
Try testing a new catalog. Import a single photo and see if you can get the shortcut to work there.
I discovered what my problem was. When I try to enlarge the image to work on it, my keyboard shortcut (Command+Space) is changing my language. And in other languages the keyboard shortcuts for Lightroom don't work in the same way. I have removed the foreign languages from my menu and now this isn't happening.
Now I just wish I could figure out how to stop that Command+Space conflict with OSX.
Many thanks for all the responses!
Go to the system preferences for Mac, choose keyboard go to the shortcuts tab.
On the left select the spotlight and then uncheck the option for the shortcut on the right window