My vizio displays no input signal when I connect it to my gateway?????

My Vizio isn't responding to my Gateway. I had a previous monitor that didn't display with my PC either. I tested my monitor on another PC, it worked great. Then, I traded out the monitor, no signal. I sent my PC in to get serviced. It passed all tests and diagnostics. They couldn't find anything wrong. Now, I have a new Vizio, it displays a little box that says no input signal. I don't know what to do. I use the VGA connection port, with Audio.....nothing!!!!! Can someone help me?????

Trini