I'm going to assume that when you say "apps from the Cable Box" you're seeing the Roku Home Screen with its apps (or "channels" as Roku refers to them). Go into Settings->System->Power->Power on and select the input for the cable box.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.