Adding the books to a new library as Kovid suggested should re-import all the books currently in your Dropbox folder .
In future, it is
absolutely critical that you do a complete sync
before and after you run calibre -- and pause Dropbox while calibre is running.
As long as you do that, it should be perfectly safe. I have been doing that for a very long time, and my database remains very much intact.
And it gets synced between computers.
You just need to be careful, that's all.