Got an Answer?
You can also get that alert if something is shared by someone from a person you are not friends on FB with and they have their settings set to view by friends only. It doesn't necessarily mean you are blocked, you might just not be friends with the person who posted it originally. :-)
Might be a link to a private group or a photo from a private page.
So I'm guessing someone sent you something? If that's the case it means that the thing they sent you isn't available for sharing and whilst they can clearly see what they've sent, you can't.
If it's come about because you're trying to reply to a message and the person who originally sent it is no longer visible as the sender, it's because their account has either blocked you or been deleted.