We've been experiencing this problem at the office for a while, on three different machines so far. CPUs are all Fujitsu Siemens with Windows XP installed, monitors are either Diunamai or Dell.
The problem can occur at different times on each machine: when you start the CPU, right after you've logged in on Windows or even while you're using the computer.
What happens is that, all of a sudden, the monitor stops receiving any signal from the CPU, which is still working, and just displays the "no signal" message.
In addition to this though, the CPU becomes impossible to turn off unless you unplug it, i.e. cut off its power. Pressing the reset button won't work anymore after the CPU's stopped sending signal to the monitor.
Now, one of our technicians replaced the VGA cable on one of those three machines and found out the computer is working again, but that can't really be the source of the problem, or that thing with the reset button wouldn't happen each time, right?
One computer showed this problem whenever I would log in on windows with my account and mine only, but not with others'; then the problem extended to other accounts on that machine; how can it be really solved just by changing a VGA cable?
Another of the machines affected used to stop sending the signal to the monitor every now and then while used, but it was still possible to work on it for some time. Back from vacation, I found out that pc wouldn't even turn on at all(a green light would blink for a while after pressing the power button on the CPU, then it'd go off with the whole machine); and today, after having obviously not being used for days, the CPU DOES turn on again, but isn't sending any signal to the monitor (again, pressing the rest button won't work).
I've run Malware Bytes on at least one of those three machines and haven't found anything (that's not our main antivirus, but it proved useful in the past to get rid of some rogue malware and trojans that mcAfee wasn't able to detect).