Welcome to JavaRanch. Well, you've lost all files, so if those contained any changes from whatever JForum distribution you were using - those are gone. There should be no problem to install JForum again and hook it up to the existing DB, though.
In this case "installation" would mean something manual done by you, not something that could happen automatically by the installer that's part of JForum. The easiest way might be to grab the latest war file from http://code.google.com/p/jforum2/downloads/list, and deploy that.