Where did my database go?

I was rather alarmed that having spent hours creating my database, when I go to File-> open recent database -select my Databse - it opens an empty database.
The only way o retrieve my database is to restart DevonThink and it opens as the default database.
I am now worried that if I create a new database I wont be able to get back to this first one.
Please advise as I dont want to have to recreate this database all over again…

Are you sure that it did open an empty database? Assuming that DEVONthink Pro was still running and that you didn’t close the database on your own (via File > Close Database), then choosing an already opened database should only open new window if there’s no window.

Anyway, a window opened via File > New Window should definitely show your contents. Otherwise please check the statistics of File > Database Properties too.

My confusion was that the name of the file on the HD was different to that of the database.
Is there a way to do ‘SaveAs’ so I can rename the file on the HD ?

No. But you can rename the database file (*.dtBase) if DEVONthink Pro is not running of course.