At the very bottom of your stats (on the account page) is a legend describing the various symbols.
If the 'H' is removed, your hub is no longer featured. This also means it is no longer in circulation for the search engines to find. This can be reversed by editing your hub. Review your text, titles and sub-titles. Once you are done editing the hub will be automatically reviewed (takes about 24 hours) and may possibly be returned to a featured hub status. If it is a featured hub, you will see the 'H' again.