[gokce]

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.

[Adlai]

If you have some technical savvy, try http://extundelete.sourceforge.net/ for ext3/4 filesystems, http://ntfsundelete.com/ for NTFS.

If you're using something other than than one of those two filesystems, you should probably know how to handle this yourself.

[Samuel_Michon]

You left out HFS+, OS X's file system.

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