If your network or other clients on the network crash while working with Mediator in multiuser-mode, the project file will be locked. This happens because the client(s) lost connection to the server.
To re-enable the document, delete the MyProjectName.multiuser folder from the same directory as your .md8 file. When deleted, open the document in single user mode and step through the pages.
Close the project after stepping through the pages – after this the project can be opened in multiuser-mode as you normally would.