Shoutbox

Does it happen for all games? Or just a particular one?

Are you sure the cables are connected properly?

Are you using a mode on the xbox which your monitor/tv doesn't support? ( On the xbox dashboard, go to the System tab > Console Settings > Display to find the current settings and mess around with some others)

If the xbox dashboard is displaying fine and it's only when you start a game, I'd suggest it's something to do with the Display settings.