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
iOS 7 & Mac OS X 10.9 Set For The Same Release Date ?
With OS X Mountain Lion and iOS 6, Apple was clearly trying to bridge the gap between both iOS and Mac including similar features in both operating systems such as Siri diction and notifications centre.
With Apple suggesting that they are still going to be concentrating on merging the two operating systems then of course it makes sense that Apple will not release Mac OS X 10.9 in the same time frame as Mountain Lion as that would reveal some of the features in iOS 7.
A lot of people have been worrying over Mac OS X 10.9 recently after rumours suggested engineers from the Mac team had been moved over to iOS 7. This could have simply been done due to Mac OS X 10.9 being complete.
Apple had promised that we would see a refresh to the operating systems on a annual cycle so we have got nothing to worry about and it is pretty much guaranteed that we will see both iOS 7 & Mac OS X 10.9 sometime this year but due to Apple not wanting the features to be revealed by the similarities it makes sense that both iOS 7 and Mac OS X 10.9 will get revealed on the same day / event.
Apple released Mac OS X Mountain Lion on July 25 for $19.99 but Apple released the developer preview on February 16th 2012. iOS 6 was shown of at WWDC 2012 and later released it to the public on September 19th.