About a week ago, I had a computer whose monitor would not wake up from sleep. Unplugging the power to the monitor and plugging it back in worked; it did this only occasionally, and, since it's a critical system for which down time is not an option, I told the user to keep me informed if it continued happening, and I left it on the back burner. Today, without any prior issues, a second computer (also critical system, so downtime not ok) began experiencing a similar, though slightly different issue. This machine's monitor, however, would not come back without rebooting the machine. I plugged a different monitor into it, and this also failed to detect a connection to the computer. I rebooted the machine, put the original monitor back on it, and set the display to never turn off, hoping this would get us through the day. A while later, the monitor turned itself off and would not wake back up; luckily, this time unplugging the monitor's power and plugging it back in worked, like the first monitor which experienced this. I switched out this monitor for one of my own (oh the sacrifice! I have to work with one monitor now) and told the user to let me know if it experiences the same issue (it hasn't yet, but it hasn't been very long).
Strangely (or perhaps coincidentally), these two machines experiencing this problem have almost identical monitors; one is a Dell E198FPb, and the other is a Dell E196FPb. These are the only machines in the building with this sort of monitor. I have tried switching out monitor cords, and, as far as I can tell, the problem is not with the monitors. Both clearly work when they detect communication from a computer. The first machine is a Dell Vostro 230 with windows 7 32 bit, and the second is a Dell Vostro 260 with windows 7 64 bit. Both are using onboard graphics. Both are running the latest graphics drivers.
I know these Dell machines have pretty meager PSUs, so I have considered this as a possibility, but the 260 is running a basic USB keyboard and mouse, onboard ethernet, sound, and graphics, and nothing else. The 230 has the same things plugged in plus a printer. Neither is reporting any resource conflicts with the graphics in device manager.
Ordinarily, I would try rolling back the graphics drivers or just uninstalling and reinstalling, and, if this didn't work, I would start attempting a series of diagnostic tests, but, since downtime is not really an option, I want to try and learn as much as possible first. A quick google search for "monitor not waking up" or "computer not communicating with monitor" brings up a plethora of possible fixes, but, if anyone can help me narrow things down a bit based on my particular situation, that would be extremely helpful.