What does it mean if your computer keeps going to sleep on you?

It started early this year. It was working perfectly fine other than, occasionally telling me it didn't have enough memory to have all the tabs open that I was using. As I'm mostly using it while sitting here on the bed, every so often, I'll need to lie back for a stretch out and will usually fall asleep for at least 20:00. When I wake up, I find the computer has gone to sleep! In other words, nothing will reactivate it!! If I push the power button to restart it, all I get is a flashing light on the keyboard and the mouse until I release the button. If I leave the computer alone for a few hours to week or so, it turns itself back on and I can use it with no problems for another few days to weeks until it falls asleep again and I can even pick right up where I left off. All the tabs are still open, I think the only stuff I have to re-open are any files I had open. It ALWAYS does this while I'M asleep or not in the room with it so, I have no clue what's going on to cause this. I've closed over a dozen tabs and usually try to have something (music, a YT video, a DVD) playing on it while I'm out of the room to keep it from falling asleep but, sometimes, it still does.
I've even unplugged it for a while to give it a fresh start when I plug it back in but, it still won't wake up until IT'S ready to!! And, amazingly, I've lost none of my open tabs or work!
Most recently, it shut itself off at least 4 weeks ago and I'm still waiting for it to wake up, again.

What does it mean if your computer keeps going to sleep on you?
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