Error after changing the URLs in General Settings

I’ve got what is probably a major rookie issue: While experimenting with WordPress, I changed both the WordPress Address (URL) and the Blog Address (URL) by adding /blog to the end of my domain name in each text entry box. Now that I did that, I can’t get back onto my website. If I type my original domain name into the navigation bar, I get to a very basically formatted version of my home page, but anywhere I go from there gives me an Internal Service Error screen. How can I go back to the way things were? I just want to be able to use my original domain names and everything how I did before I changed the URLs in the General WordPress Settings screen. Any help is appreciated. Thanks.