Changed Permalink-Site Crashed

Good day,

I thought I understood what I was doing, but I obviously didn’t. I tried to change the permalink on an old blog post in the edit post screen so that it made more sense for that particular post. Instead, I apparently changed the URL for all posts. My main page (jmtbook.com) is still there, as are my other pages. My posts still appear on the combined “blog” page. But if I attempt to get to an individual blog post, all I get is a 404 error.

Is there an easy way to get back to where I was before I made this blunder?

Thanks in advance for saving the day, Ray