since dropbox files are also stored locally it is possible that the files are still on your harddisk somewhere but unlinked. if that's the case I suggest you turn off your computer right away and see a professional.
If you're using something other than than one of those two filesystems, you should probably know how to handle this yourself.
However, Dropbox on OS X has a cache file at ~/Dropbox/.dropbox.cache – so if OP uses a Mac, chances are he won't need an undelete utility.
More info: https://www.dropbox.com/help/296/en