Why isn't my touch screen working after my run?

So I went for a run and had my iPhone 4S with me. I had it between my sport bra and myself so I could listen to music with my headphones. When I got back from my run and I was suppoused to turn off the music my touch screen didn't work. And I've tried to reset it a couple of times but nothing seems to help.

I don't know if the fact that it got a little bit sweaty had something to do with it. But I find it hard to believe because I always have it there, and I haven't had a problem with it before.

What should I do? It probably needs to be fixed, so how much do you think it will cost to fix it?