Page suddenly 404

The following began about a week ago for no apparent reason. I upgraded to 3.8 and it’s still happening. A page that has worked perfect since it was created in 2011 suddenly does not work. Nothing has been changed on that page in ages. As far as I can see absolutely everything is in order. The page is http://garlandpipeorgans.com/music/.

I copy and pasted the html content of that page into a new page, http://garlandpipeorgans.com/music-2/, and substituted that for the Music page in the main menu of http://garlandpipeorgans.com and it works perfectly!

So…the page content is not the problem. Also, the permalink at the top of the edit screen for the page that suddenly stopped working is correct: Permalink: http://garlandpipeorgans.com/music/

I am completely mystified. Any help would be greatly appreciated. I don’t know php so please explain like I’m a 5 year old.