I had this issue recently too, and doing as mont said seems to have worked for me. Not quite fully working though from what I can tell, as only some of my activity has started showing again, not friends stuff. But only sorted it out yesterday, so there might not be anything from anyone else at the moment.
Anyway, it won't update with what you've missed in that time. It will just have start working again from now. I had to earn some trophies to get it to start showing anything. Even playing a new game for the first time wasn't showing until I I earned a trophy