When a member changes their theme, the links dont work

When a member changes their theme, all of their posts are gone. Like if i go to activity and click on a rrecent post, the theem will display a page not found. i checked permalinks, it seemed ok. is there anpther reason anyone can think of that would mess up the blog links like that?

I reactivated the old theme and the links were working aagin.

hmmmm so what would cause the links to break when i change themes?