In this case and as a first step, make sure the network light of the Slingbox is on.
Besides that, you can reset the Slingbox and set it up once again. Also, make sure that the Slingbox is connected directly to the router, not using a wireless bridge or alike, just to troubleshoot.
Also, make sure that the router is up to date and assigning an IP address to the Slingbox.
If the problem is still the same after that, you can post the results here.