Apparently patch 2.0.5 came out overnight last night or early this morning. A lot of users are getting an "Unable to validate game version" error (pictured above) along with a myriad of other connection problems. The official forums are also down, or so slow as to be effectively down. I'm going to guess that perhaps your game version is fine and this will sort itself out in the next few hours as the patch gets distributed.
Thanks to everyone for submitting this as a topic suggestion, there's too many of you to credit directly.
UPDATE 10:54 from the in-game Breaking News:
"The 2.0.5 client-side patch has caused issues with our authentication system. These issues can cause players to be suddenly disconnected from the game, as well as prevent them from logging in to the game. We are working to resolve this issue as soon as possible and will provide you with updates as soon as they are available."
The realms are accessible, or at least mine is. The forums are also back online and usable.