Where are you located, and what garden zone are you in?

I'm in Los Angeles. I believe that is zone 8 or 9

Then I would be tempted to cut the stalk 3 inches above ground and leave it there to grow (or not to grow) in the spring. Or you can attempt overwintering the tubers inside, but that is not easy to do as a newbie.

The tubers are probably just fine, but something affected its top growth... perhaps not enough water, mites, over-fertilization, etc. If you've been watering regularly but the stems and leaves have been turning brown, perhaps there is a fungal issue. It is really hard to tell from the pictures.

If you just have a few dahlia, there's not much invested. Don't sweat it, and add to your collection in the spring. Normally whatever ails the plant leaves the tubers alone, so perhaps the winter dormancy will cause the problem to go away. (wishful thinking, perhaps, but it happens) Hopefully next year is better for you.

I read about doing that. I might. It might've been lack of water. We had a really hot Summer here. A lot of things got scorched. Thanks for your replies. Sorry for taking so long to respond.

Yup, lack of water will definitely cause what I see in your image. Once the roots are established and there is top foliage, dahlias like constantly damp, well-drained soil. However, undrained soil will cause rot.

I water at least three times a week, with it raining one or twice besides. Show folks often water a little every day.