I just installed my Roku device and it seemed to be working great. The problem came when, after watching my movie, I tried to switch back to watching regular tv via my Comcast cable connection. When I click on the video input button to switch back to cable, now I cannot get regular television.
As the responses say, this is probably an input selection issue rather than a Roku issue. If you post how you have the Roku box and the cable hooked up, particularly if and how you run it through an A/V receiver, and what the model TV and receiver you are using, I think people here can sort it out.