Some pages are showing 404 error, even though they were previously working.

I don't know what has happened here, but when I click on a product (portfolio item), I get a 404 error even though I checked to see if the page was there.

If you click on any one of the products here you will see what I mean.
http://tinyurl.com/p5ztgrw

How can I get it to go to the page?