Occasionally when working with a group of developers, someone (and I'm not going to name names...) will accidentally delete an entire directory of the subversion trunk and commit that change. Luckily, it's not the end of the world if this happens. Change directories into the root of your subversion source tree and issue a merge command as follows:
svn merge -r [bad revision]:[good revision] .
svn merge -r 13831:13830 .
After that, checkin your changes, and you're back in business. You can determine revision numbers by doing something like:
svn log -v . | less