The "No Signal" message means whatever source you're using for the TV to display is either not connected or turned off. There are an awful lot of reasons this might have happened; here are a few off the top of my head.
* Cable has come unplugged
* Source was turned off
* Wrong input source is selected for the TV
Check the cable connections and make sure whatever you're using (cable box, game system, streaming device, etc.) is plugged in and turned on.