I'm guessing you're using an HDMI cable. If that's the case, I suggest you try using a new high speed HDMI cable version 1.4. Try not to settle with the cheap ones as this cable is complex. Some people would say that it's just a simple cable but there's more to that.
My Samsung TV had been working perfectly fine until last to last night and yesterday morning when i switched it on, its displaying a message "no signal or weak signal". Its a smart TV with integrated cable card receiver. I've have also got my TV checked by the cable company and they say that there is no problem with their Card r CI-model. After that i tried the automatic channel search and the TV started displaying a message that there are no channels to found. I have searched the internet for answers but no one really knows what to do, even though a lot of people are facing the same problem.