A few days ago, I noticed that when searching on SO, results flash for less than a second, and then disappear. Using both Firefox's and Chrome's latest stable builds on Windows XP causes same result.
I am guessing all the search results share a tag in your Ignored Tags list with your preference set to "hide".
Try doing the search while logged out, or from another web browser, to confirm this.
Or you can disable hiding ignored tags, obviously.