Some hundred years from now, humankind has mastered the secrets of extreme longievety. They can cure all illnesses, stop aging (maybe even reverse it to a certain extent) and replace any broken parts. Humans can therefore be, well, extremely old. From our point of view, they would be immortal. They'd live not just thousands of years, but tens of thousands of years - the only real problem being how long the human mind can cope with living before growing too bored of the world.
The people who are first able to undergo this treatment are the rich ones, and the great scientists who made the discoveries. That is to say, well, "Important People".
Before all of humanity are allowed to undergo this treatment, however, war breaks out. A huge war. One involving, well, all of Earth. The agressors might've been people who hadn't been allowed to take part of the longievety-technology just yet, though I'm not quite sure about the reasons behind the war, only that it breaks out.
Some humans manage to get away and shield themselves off from this. Of course the people with the highest chance of this are the "Important People" - rich people who can buy safety, scientists, the like. Most of these who have a near eternal life expectancy span, so it makes sense to save them, doesn't it? Because they will be able to rebuild civilisation if need be.
So, a few hideouts - I'm thinking smaller cities underground/inside the mountains, that have been specifically constructed to keep radiation out - are saved from all ill effects of the wars. The people inside have food and water, they have the means for getting more, and... yeah. They are self-providing. I will from now on refer to these "Important People" as either "elves" or "gods", as that is what I'm intending for them to be like.
The rest of the world, however... is pretty much destroyed. Nuclear bombs are dropped all over the place and humanity almost dies out. They, of course, loose most of their current level of technology - and civilisation declines.
With all the radiation out there, mutated children are much more frequently born, and some of the mutations provide them with extra benefits, which makes the mutation live on and... yeah. Evolution kicks in. (I am not really a die-hard evolutionist, but it works so well for this setting!) Some thousand years later, humankind has been divided into several humanoid races - perhaps some of them have "gone back" the evolution tree, and become really smart monkeys. Some of them have grown wings and learned to fly (angels! ), some of them have turned into giants, and some into dwarves or hobbits .... maybe some of them have even developed gills and can live underwater, now.
So. Yes. This is the world I have - the secluded people who will live forever, and lots of humanoid, intelligent beings. I imagine civilisation will be on the rise again in some places - think about late bronze age/early iron age.