Actually, if I'm interpreting the series of events correctly, the TV/Monitor has been switched to an input port that isn't currently connected to anything. Most TVs and monitors I've seen will maintain the input setting through a power cycle. You have to manually change the input back to a port that is connected before you'll see anything. (Unplugging might work, though...)

I would love to get confirmation from someone whether the Input key is not letting you toggle back to the original input port on that TV. I'm not absolutely sure at this point whether the key means different things to different LG tvs or not...