|To prove that it is stuck in power saving mode, press and hold the power button until it shuts down. when power light goes out, release it. Press it again to restart it. If it starts normally, it probably was 'stuck'. Go to power settings, set Hard Drive to 'Never', Monitor to 10 minutes, Sleep to 20 minutes (for desktop; 'Hibernate' though if laptop), and Apply. See if these settings prevent it from happening again.|
I have seen this happen before and it appears to be that the Hard Drive it set to power down at the same time or before the Sleep and appears to cause this. Since the hard drive would already power down during Sleep, it is redundant and appears to prevent the saving of information from the memory to be saved to disk.
If this happens again, try to disable all except monitor powering down and manually use the Sleep feature (Start>Shut Down>Sleep) when you will be away from the machine for more than 15 or 20 minutes.
You have to be a little bit crazy to keep you from going insane.