Can’t Log In! Internal Server Error Message…

A few hours ago I attempted to change my blog’s domain (as in going from http://currentdomain.com to http://newdomain.com). I didn’t know how to do that, so I experimented by going to my General Settings and changing the URL/Home location there. Obviously THAT WASN’T THE WAY TO DO IT, as I haven’t even been able to log in since. I understand NOW that I should’ve changed something else in the files first, before changing it in my Settings.

Anyway, the point is that I can’t log in now. So I need help with:

1- Changing it back to how it was so that I’ll be able to access my account again;
2- Changing domains THE RIGHT WAY, which I’m still not sure how to do…

PLEASE HELP!

P.S. I’m NO computer guru so I’d appreciate if you kept your instructions as EASY AND SIMPLE TO FOLLOW as possible. Thanks!