Who knows? Depends on how sharp your coding skills are. If you have no idea where to start, my best guess is that it will be a “learning experience” for you, and maybe fairly difficult.
You need to provide more details. What kind of software do you use? WordPress? another CMS? a “home-grown” script? How are your messages stored? How is your “admin section” designed? What programming language does it use?
*Assuming* your messages are stored in a database, there is likely a field in the table which indicates the status of each message. Making a message “unread” again would simply be a matter of changing the value in that field. As I said, however, it’s not possible to give a more useful answer without more information.