Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Apple Release Messages Beta To The Public , Basically iMessage For Mac –
Earlier today we got an unexpected announcement from Apple about the next addition to the Mac OS X. They introduced Mac OS X Mountain Lion , It was available for developers today and is coming to the public in the summer. A big feature in OS X Mountain Lion is Messages. Messages is basically iMessage for Mac. Apple has joint there current iChat with iMessage to create messages. The good thing about messages is it is free to download the beta today ( Download link below ). From my experience with Messages I can see it becoming a conversation hub and in the feature it including emails , tweets ,etc.
Download Message Beta : http://goo.gl/oyXqw