You have a furnace somwhere in the house, and it has a vent pipe that most likely goes out through the roof, running through something called a "chase".
In fact, you might very well have more than one chase, depending on how the vent stacks for your plumbing are laid out.
What's nice about these chases is that they are open from the attic to wherever the furnace is.
The most effective way to get cable through there is to attach a few washers or a lead fishing weight to a thin cord, then "fish" it down through the chase.
Take a look and tell me what you've got and we'll take it from there.
And don't worry...
Even if your house isn't laid out in such a way that the chase solution works, there are other ways, such as methods to drill through the floor INSIDE a wall.