I have an "Out of Range" error on my plasma. What does this mean and how do I correct it?

I have a Pioneer 50" PDP-503cmx Plasma with BNC connectors and BNC adapters to run the RCA GBR cables for viewing. It also supports VGA. Along with the GBR connections it has HD and VD connections. Inputs 1 & 2 are VGA to DVI for input 1, GBR, HD & VD for input 2. When I change from input 1 that displays my computer to input 2 that should show my HDTV it flashs an error of "Caution Out of Range" and then flashs off. When I try an access the onscreen menu to change settings it won't allow me to do so, but when I change back to input 1 it will. Any ideas? Thanks.