Weak or no signal

Question

The other day I decided to add a cable box so I could watch On Demand through Comcast. Once I hooked it up the message on the screen said ''Weak or no signal''. Comcast wanted to come out but I had decided to just return the box. When I reconnected the cable directly to the the TV from the wall I still got the message ''Weak or no signal''.I have had this TV since around April 08 and it always worked fine. It was working right before I connected the box but not since. I have also tried connecting the cable to a different TV and it worked fine. Any ideas?