Thanks for using our answers forum! I'm sorry to hear you're having an issue with your SlingPlayer for iPad. I know it may sound simple but is your cable box turned on? When I initially log into my SlingPlayer I sometimes receive a "no video signal" message until I pull up my remote and power on my cable box. Beyond that, you may want to try logging out and logging back into your Sling Account. I would advise uninstalling and reinstalling the application if the issue persists any further.
Here is a link from our support site which explains how to uninstall and reinstall the application:
Hope this helps!
The Sling Moderation Team