If an user makes a question hidden, can he unhidden it later?
Yes, they can. Also note that in admin/permissions there is this setting Hiding or showing any post that allows users with certain privileges to hide other users' posts.
Where can he find his own hidden question?
The link stays the same. This means, if a user knows the link of their hidden question they can access it directly. In the admin/hidden section there are also a list of posts that have been hidden. The thing is that, if the user is not an admin and hast lost the link to the post they have hidden, they won't be able to find it because they won't be displayed in the lists.
A better approach could be to add a Hidden submenu item, similar to the one in admin/hidden, in the user profile page that displays all hidden posts. Alternatively, in the user profile page too, the All answers and All questions could really include all of the answers or questions, even if they're hidden (they could be greyed out in the lists). The downside of this approach is that comments can also be hidden so it would need an additional menu item for All comments.