I've had this many times. What I don't understand is that I have "forever" selected on my log in time, yet it still does this.
Actually, you don't even have to restart your browser. When my session ends like this, it still shows me being signed in. Just log out, and then back in, and that should work. If it shows you being logged out, simply log back in. Make sure to copy your post first, as they said.