A couple of things, try both the Slingplayer Desktop and Web Slingplayer to see if you get a different response from either of them.
If that doesn't work you can try peforming a factory reset by following the instructions described in this article. (http://support.slingmedia.com/get/KB-1000038)
another possible reason (i know this from experience) is that your computer could be on a different network. With me, my computer had connected to my neighbor's unsecured wireless network and I didn't notice. I didn't figure out why the Slingbox wasn't showing up until I physically plugged my computer in to my basestation and it suddenly showed up.