CarrionCrow: First thought - looking at each organism, whether human or not, being a small thread, interconnected and woven into the larger, overall thread of energy that keeps everything going.
A person's thread frays, they die. No coming back. For gods, possibly strands that connect them to points that mortal races can't.
How did the gods go away? Most of their threads were cut, maybe to the point that they couldn't actively interact with the mortal world, but still bound sufficiently to still exist in some state that no living mortal could.
How does a demi-god work? Well, you get two mortals together, they reproduce, you get a new mortal thread.
A god and a mortal reproduce, you get a reinforced thread. Stronger the thread, the closer to divinity you are.
The tricky part would be how exactly the gods went away and what they have to do to come back.
First thought - someone managed to find a way to unbind the threads and did so. Why they did it? Who knows. Maybe the gods were complacent, omnipotent assholes that needed a good comeuppance so they'd stop abusing the world with their power. Or something. That's up to you to decide.
But anyway, as for how the gods come back? Well, they might have to sacrifice part of their own essence, losing connection with some things and redirecting the threads to essentially reattach themselves to the world.
Or maybe, mournful followers develop a way to ritually sacrifice themselves in order to give over their threads to their god, allowing the god to rebind themselves at the cost of their own existences.

gunsynd: Some people get insulted by our slang,either they don't understand or they too stupid to find what it means.
What they should do is a Captain Cook first...

Stilton: Good morning one and all, the Viking has crawled out of his hovel unaccountably early this fine chilly morning and is eager for fun and games. Is anyone awake yet? If not I'll have to talk to myself for a while (and I've done more of that than I'm comfortable thinking about ;-))

Stilton: Good morning one and all, the Viking has crawled out of his hovel unaccountably early this fine chilly morning and is eager for fun and games. Is anyone awake yet? If not I'll have to talk to myself for a while (and I've done more of that than I'm comfortable thinking about ;-))

CarrionCrow: Good morning. =)
Still awake here, and probably will be for most if not all of the night.
Sleep patterns - purely mythical. Also? You aren't the only one who talks to themselves. ;)

Stilton: Man, I thought you'd be in bed having dreams of Armageddon and/or needlecraft (or similar). Glad you're not. What time is it there, by the way?

CarrionCrow: Nah, no dreaming here yet. I've been chewing through Soulstorm like an enormous, seemingly neverending platter of gristle-laden meat.
Right now, It is 12:43 am in my dusty chunk of the Pacific time zone.
The time zone adjustments definitely take getting used to. Australia's the craziest by far. Gunsynd is 19 hours ahead of me, and I don't even know how far ahead Niggles is since Australia's large enough that it has multiple time zones of its own.

CarrionCrow: Course, I'm also a bit weird in general, so it bugs me to say the wrong thing.

Stilton: That sentence reminds me of when I was at art college and this drip of a 'lecturer' was trying to tell us that 'nothing was wrong and nothing was right, nothing was good and nothing was bad...' I was tempted to stick a chair up his arse and ask him how good it made him feel. Amazing that cretins like that are happily turning young minds completely off course.

CarrionCrow: Sounds like the art college was a bit desperate for speakers at the time. I can get the idea that good and bad are subjective things depending on perspective, but at the end of the day most people (unless they're completely out of their gourd) can list an array of actions that, if carried out, would make them a total and utter asshole.
Based on that simple knowledge, I think it's safe to say that such actions can be considered "bad" without worrying about oversimplification.