The word ''no sync detected' means that there is no video connection between the cable box and the TV set. Isolate the source of trouble by connecting to the TV a prevoius video source that worked properly e.g DVD player. You should also try to get a new set of video cables as the ones you have may be internally broken.
Please follow the installation instructions carefully especially the colour codes of the video and audio cables i.e yellow cable to yellow connectors of TV and cable box , Red cable to red connectors of TV and cable box etc.
good luck!