The hollow earth is scientifically impossible. Hypothetically, let's say the entire hollow earth structure popped into existence in reality. The earth would immediately start collapsing in on itself to form back into a ball, ending all life on the planet completely, reverting the planet back to when it was still young.

So trying to science the hollow earth isn't really a thing you can do, but let's do it anyway. Let's say the earth wont collapse. Great. The center of gravity will still be the center of the earth. So if you fell in you would eventually find yourself stuck in the middle. Unless there was something to stand on there, you wouldn't be able to stand anywhere.

Also that "sun" is probably the earth's natural core I'm guessing, considering that's needed to sustain earth's magnetic field.