The screen is black and a little box that says no signal is moving around on the screen

Check your inputs. The no signal box means exactly that - there's no incoming signal for the TV to decode. So you've either disconnected a cable somewhere or turned off your input device.
Try changing the inputs on the TV (this will be a button saying input or A/V)
Try changing the channels
Check that your cables are correctly connected and that whatever you are trying to use (dvd player, PS3, etc) is turned on.
Check that your TV is tuned to the corresponding input it is plugged in to (if you plugged your DVD player into HDMI input 8, tune your TV to input 8)
Hope this helps.