Check you standby and hibernation settings. It might automatically go to standby after a certain amount of time, then hibernate after a certain amount of time on standby. That's what my laptop does.
I cannot check anything that uses power as my computer is a paperweight. There is no power to check standby mode. I did pull the switch off the front to check it, and it seems all right. Again, I changed the power supply, which very briefly kicks the fan but immediately stops.
