This exact same thing happened to me today, the Help Viewer would crash immediately upon opening, without an error message or anything (to my knowledge).
A search on this here at MacNN seemed to indicate it's pretty common, with several saying that the Help Viewer is the worst piece of software ever written.
What worked for me was to delete:
username/Library/Documentation/Help
username/Library/Preferences/com.apple.help.plist
Restarted help and all seems well so far.
Hope this helps,
Mike