If the forum has actually been abandoned, you can ask Network54 to assume ownership. The forum is considered abandoned when the forum owner has not logged in to Network54 for at least 6 months. They do not have to post or moderate the forum in that time, they just need to log in.
If the forum is not abandoned, you can start your own forum and alert members to the new location. This can become a problem if the current forum owner gets upset by you "stealing" their members. If they truly are missing, however, they won't mind.