My LG Flatron W2452T is usually left on "stand-by" mode after I've turned off the computer, and on re-booting, it has always started up normally. Recently however, I'm having to switch it off, then on again, before the screen appears.
Possibly 'soft start' circuitry on your LG Flatron not responding for whatever reason. Have same set up and had similar problems. Try unplugging and reconnecting both power lead and vga connector in case its a bad contact. Have there been any 'arching' sounds from your power lead on the back of the monitor? A long shot but quick fix if it is this.