If your Slingbox isn't displaying a picture, it is possible the set-top isn't providing a signal. I would recommend testing the connectors from the cable box to see whether or not they're working. It is possible that the set-top box has Selectable Output Control (SoC) enabled, which is a part of High-bandwidth Digital Content Protection (HDCP). If your set-top box is also connected so another device using HDMI then its probable that it has disabled its own analog outputs.
There's more information and a possible workaround in the articles below.