Most likey there's a problem with your set-top box. Since, it sounds like you're getting audio and video from the information channel. Could it be possible that the box is connected to a TV via HDMI? Several manufacturers shut down the analog signals while HDMI is connected. Or maybe, did you forget to pay your television subscription service?
Probably the best test is to connect the bypass from the Slingbox's audio and video output to the TV. Or check to see what is comming out of the set-top by connecting it directly to your TV. Because the Slingbox can only rebroadcast whatever it receives.