Issues with <?php the_ID(); ?>

I am trying to call the ID of a post in the side bar. This works on the home page, displaying the ID of the latest post. But when that posted is visited, the <?php the_ID(); ?> no longer displays the ID.

I don’t know why the ID is called on the home page, but not on the post itself. Is thereanything I have done wrong or am missing?

Thanks for any help you can offer.