You may want to read your server's disaster plan. That is, if you ran a server, you as a professional would have a plan for when the server melts down. Time to get out your plan or if there is none, consider this the lesson why your next server will include the disaster plan.
It is possible that the database no longer exists or is part of a database mirror. To avoid the error log on as a user who can modify logins. Then, change the user's default database to a database that is currently available for a connection. If it useless try this application MDF Viewer Tool https://www.viewertool.com/mdfviewer.html