I have had three systems that did this. After about 22 years of hardware woes, I have found at least one solution although it might not be yours.
One of the motherboards was high enough to intersect with the drive bays, apparently, due to crappy surface soldering and drive insertion. The capacitor up there got 'clipped off' and it didn't even appear to be missing, since there were plenty of places where parts were left out due to features being withheld from that model. It wasn't until I took it to sell the board when I found this out, and looking closer it was obvious because the capacitor was sitting at the bottom of the case in the corner (one of those mini silver caps).
After reconnecting the "fallen cap" the board (which the store wouldn't buy due to physical damage) now does not go into "snow white mode" anymore.
Possibility #2
This ASUS motherboard I have does this as well, checked for missing parts but that was fine--found out it was due to extreme heat, the inner layer of the board had been warping causing some traces to possible be disconnected which eventually ended the board as well--this was due to a CPU fan failure. Also, you might check the board & CPU temp immediately after rebooting when this happens.
Many systems overheat and it wasn't until I was running Linux that I found out that the system was shutting down just after CPU overheating not due to dirty fan but just too many times had I cleaned it, disconnected the CPU fan and heat-sink and over the years, the heat-sinking compound had become so thin, that overheating was happening between the heat-sink and chip, the solution there was getting a tube of heat-sink and that helped greatly (-20deg!!).
Possibility 3: USB
It was a faulty USB port that was the cause of my problems (waking up). I didn't use the USB on the back, and apparently its previous owner had messed up the pins. Because of this, I suspect that it wouldn't come back from sleep. I had to disable USB altogether and use a PS/2 keyboard and mouse, then it was fine. Now the other systems were pretty much like the three I mentioned, so I'll skip that.
The bottom line is:
- 90% = hardware problem
- 5% = software problem
- 5 = BIOS problem
To this date I have never in the past 22 years EVER had a BIOS flash fix anything (however, I did have a few break PERMANENTLY afterwards!).
I know you said the computer wasn't set to sleep, but what does the computer power button look like during this time? Is it solid-on, or is it pulsing? – Kruug – 2013-02-01T14:44:10.643
It is solid. Yeah, the blinking light would be a dead give away – VoidKing – 2013-02-01T14:45:38.547
What about the light on the monitor? Maybe it is a monitor setting, to turn off after a while. Try pushing the power button on the monitor to turn it on, see if that works. – BenjiWiebe – 2013-02-01T14:47:12.190
I have and it just gives me the "no signal" message. When this happens I literally have to hold the power button in on the machine until it turns off, then turn it back on to get display back. – VoidKing – 2013-02-01T14:48:04.897
Is it a multiple-input monitor? Maybe once the computer turns off the display, the monitor is switching it's inputs. – Kruug – 2013-02-01T14:57:46.513
Problem is, this could be anything. Most likely the code has wandered off into a corner (perhaps due to bad memory, perhaps a software bug) and is not responding to interrupts. But I had one computer where similar symptoms were due to the monitor -- it was apparently giving back bad info when identifying itself during boot, and so once turned off would not turn back on. – Daniel R Hicks – 2013-08-04T19:28:35.970
@VoidKing : Are you able to swap another monitor and then check the same. – Renju Chandran chingath – 2013-10-03T08:11:53.900
What kind of cable(s) are connecting the monitor to the video card? What port to what port? It's possible that a DVI-HDMI adapter may be faulty. – Bigbio2002 – 2014-02-04T01:57:53.293