Which Slingbox model do you have? Most Slingbox's require a factory reset and reconfiguration. The Slingbox 350 may just need to be power cycled. The Slingbox M1 or Slingbox 500 can be manually told about the new network. Below I have linked some articles that should help:
- How to change the network settings on your SlingTV/Slingbox 500
- Changing Slingbox M1 network configuration
- How to reset your Slingbox to factory settings
Hope this helps!