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 Launches Updated iCloud Website For Developers
Apple has today launched the new iCloud.com beta for registered iOS and Mac developers. The new features can be accessed at beta.icloud.com and this was leaked prior to WWDC 2012.
The new updates do not include many features, two changes however include, notes and reminder applications for the web, a Find my iPhone app and an improved Calendar application!