Everything stopped working when I uploaded a theme

I downloaded a theme called Quill. Once I did that, I imported some files to make my front page look like the demo. I decided that I still didn’t like it, so I uploaded a new theme to try called Launch Effect. As soon as I did that, every thing I clicked on my dashboard took my to a page that says “Internal Server Error.” I proceeded to exit out of everything, and I tried to re-login to wordpress, but the same page saying that I have an error showed up, so I can’t even get on to my dashboard and delete any files or themes. Please help as soon as possible.

http://www.sextonmediagroup.com