No signal!!! That's the kind of warning that's more associated with a lost WiFi or HDMI signal.
Are you connected by WiFi by any chance. If so, are you able to connect to other internet based services, such as BBC iPlayer?
By-the-way, in the interest in collecting information. What's your LG televisions 'model number' and 'Software Version'?
What's your NOW TV 'app' version?
Have you tried your LG televisions 'Intialization of App' option (if available)? Which can be found via: Support '?' -> Initialization of App...