I have the same problem. My database dissapear for the 3rd time and I haven't deleted it!!!! My data is posted on dropbox as I use it from iphone and desktop. It is the third time it happens to me - I make a lot of changes and instead of saving data, the whole file get deleted and then I have to remember....
Markus, look at the recycle bin, although my changes were not saved since the last successfull save