As you're working, Site Solution shows you just how your site is going to look to your site visitors after you publish. As such, you won't be able to use your site menu to open a page you have removed from your navigation, because that page will no longer appear!
To edit a page you have unpublished, you'll need to use the Manage Pages dialog:
YSS Site Pages Button
YSS Manage Pages Dialog Menu: Edit Page Button