Since you’re dealing with a post, the “page.php” file has nothing to do with this.
This looks like the usual “single” post page which doesn’t include the sidebar. When you click on a title in a post, it uses the “single.php” template which is based on “widecolumn” css settings and obviously doesn’t have a sidebar call in it.
So looking at your page.php code isn’t going to help you.
You can modify the “single.php” template to add the sidebar to single posts. Just make sure the the content class is the same one that index.php uses. Index.php is what your normal blog pages are based on.
I’m basing all these statements on what the default theme does, so if your theme is similar to the default theme’s structure, I bet this is probably what you have to do.