Sounds like you're running into what a lot of customers who use DirecTV have come across. It sounds like your cable provider may have updated your cable box with content protection, which shuts off the video signal it gives to the Slingbox. Here is an article that discusses this content protection:
Basically, there are two ways around this:1) Disconnect the HDMI cable from your cable box completely, and run a second set of component cables from your Slingbox to your TV (don't worry, you still get an HD signal).2) If you wish to keep using HDMI with the TV, and still be able to view your Slingbox using component cables, we recommend the use of an HDMI adapter. Here is some information on that:
If neither of the above gets video back on to your Slingbox right away, try giving a quick reboot to your cable box.
Hope this helps!