was it ever working?
could be the cable is hooked-up to the antenna input or the tv is just set to the wronge input. On my 3-4 year old samsung lcd tv there is a button on the remote labled 'Source' to change the input, if samsung kept that on there remotes, have her click on that until is gets to the cable input. If still nothing could be a the cable.
Trying to figure out why a brand new tv I purchased for my grandmother would be displaying a "No Signal" message on the screen as well as not displaying any cable image.
The tv is brand new it's a Samsung TX-T2782 tv and it's connected to cable without the use of a converter box. My guess is that she's pressed the wrong button by mistake on the remote control. Problem is she's calling me at work and I'm not there to be able to try and fix it.