The only way to update the firmware is through the Slingplayer software. The only way the Slingplayer software can access the Slingbox is if it has a valid IP address. I recommend in attempting a factory reset on your Slingbox (http://support.slingmedia.com/get/KB-1000038)
If that doesn't work its possible the Slingbox has a hardware problem described in the following topic: (https://community.sling.com/message/4194). In which case you should call Slingmedia technical support, walk through their troubleshooting steps, and if they find it a hardware issue then they'll send a replacement.