This is out of pure curiosity, but how can an entire member database disappear?
Actually there can be many different ways this can happen.
The one Larae mentioned about running a repair and optimize function when the server decided to be weird is definately one of them.
Other reasons could be, the hosting company that has the site and database did an O/S upgrade, a web server upgrade, a database upgrade...any of which could cause a database to do weird stuff.
Server restart while the database is performing some operation can do it to.
Something else happened while the DB was doing a write, or delete action which overwrote or deleted everything..
etc...etc...etc.