Sounds like its set to go into hibernation after a certain interval of inactivity to save power. I would leave it as is, it will save all your progress when resume and doesn't take too much time to get back up and running again.
If you want to change it go to power option in control panel and go to your power plan. Then advanced power setting->sleep->Hibernate after. Set it to 0 for never going to hibernate or a really large value. Then you may have to adjust some setting like brightness to get windows to accept the change to advanced option. (I think its a flaw in windows design) and then save the change