You could try deleting the cache files for Help:
Navigate to the /Users/yoursusername/Library/Caches folder (in shorthand, as you will see it written this way more often than not, that is ~/Library/Caches - the tilde indicates that this is in your home folder).
Move the com.apple.helpui folder to your desktop and relaunch Mac Help. Does it work now? If so, delete the old cache folder.