link appears to be broken when trying to login

I was in my settings and changed https://mydomain.com/blog to https://www.mydomain.com and now I cant login to admin. I get an error, appears link is broken message. When i go to mydomain.com/blog I can see text only and no wordpress formatting, headers, etc.
I was trying to just change my blog title to link to my site home page and not the blog page. What file do I need to edit to get back to normal. Thank you.