This assumes you are speaking of an LCD monitor...
Sounds like your monitor is dying (long returns from power-off, and all-white screens are bad news).
You may want to see if it behaves the same way on another computer; If so, then it's done-for.
Otherwise, you'll want to get a hold of another monitor to try with your system (including a new cable) to see how it behaves. If it's doing the same thing, it's not the monitor.
But I'd be willing to bet a doughnut or two it's the LCD, as we were fortunate enough to replace a couple/few dozen Samsung's under warranty due to this same problem over the year. Eventually (if not very soon after problems started) the white screen would be all you got.