Use Onyx to "Reset MacHelp"
http://www.titanium.free.fr/pgs/english.html
If you have suddenly lost access to the Mac OS X Help system in a multi-user setting - usually after a system update - you may want to check your User restriction settings.
Look in System Preferences under Accounts, and then the capabilities menu. If a user is restricted to use only certain applications, Mac OS X will not allow you to launch the help system, as it is treated like any other normal application.
If you do indeed intend to restrict user access to certain applications, you can add the Help Viewer application to the list of accessible applications by navigating to /System/Library/CoreServices/Help/Viewer.app in the System Preferences Users pane.
Help not working;
Go to User/Library/Preferences and trash the following files:
com.apple.help.plist
com.apple.helpui.plist
com.apple.helpviewer.plist
Next go to User/Library/Caches and delete the com.apple.helpui folder.
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Tiger
The Help Viewer preferences and/or caches file corruption may cause this problem.
Delete the following files, then restart and try Help.
HOME/Library/Preferences/com.apple.help.plist
HOME/Library/Preferences/com.apple.helpviewer.plist
HOME/Library/Cashes/com.apple.helpui (folder)