Cable TV No Signal (Is it On)

-Oh Lord! I just bought a new TCL TV yesterday. I did tighten the cable to both cable box and wall cable many times, but it won't work. TV message said "Cable TV No Signal (Is it on?). How can I resolve it? I need to make an appt with Comcast/Xfinity techician to come over at my place. I have a very hard time to find tech help support email address., but no available.