Pure fantasy, I will try to make it sound logical for you.
Imagine if you will, living at the pole in Antarctica in winter and a deep cold wave coming.
That would be just the start.
Humans could survive underground a good while, maybe even genetically morph to adapt to subterranean life.
We would be like worms deep enough the geothermal energy would be survivable, but in great time, the earth would loose too much heat energy to support life.
The sun actually does little to heating the earths core, that will be around a long time to come.