Valve released a patch a few days ago that caused Dota 2 to be inaccessible to all Linux players, but the problem has been quickly remedied.
An update has caused a lot of grief in the Dota 2 Linux community, especially on Github, where many players have pointed out the issue.
According to one of the users on Github, Valve accidentally built a game version with dynamically linked libraries instead of statically linked ones, which caused the client to miss a critical library.
The studio released a fix for the Dota 2 Linux client and everything went back to normal. You have to keep in mind that Dota 2 for Linux is still considered Beta and a lot of other problems might surface in the future. It's good to see though that Valve has responded so quickly to this issue.