Typically that's an indication that your box has failed. You may need to seek warranty repair or replacement if that's available. However as a last resort you could try doing a factory reset in hopes it was just malfunctioning. The catch is, after you do the hard reset you'll need to set up the Slingbox again as if it were new.
Resetting Sling Box
To reset your Slingbox, you must be on your home network and be able to physically access the Slingbox.
With your Slingbox powered on, press and hold the Reset button on the back of the Slingbox for five seconds.
The lights on the front of the Slingbox quickly flash from left to right, and then flash slowly.
The Slingbox is reset after 30 seconds.
Once reset, the Power light stays solid.
If your Slingbox is connected to your router, the Network light will also go on when it receives an IP address.