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
Next Generation iMac's In Production Ready For October Launch –
We are hearing reports that suggest that Apple’s supply chain is getting ready to ship components which will be used in the next generation iMacs. Some of our sources from with in the supply chain has said that there is a fairly high chance that Apple will include a retina display in the next generation iMac’s which are set to be released in October. We have not got any information on the price as of yet but it is looking likely that Apple’s iMac margins will take a hit or the price will increase a little as the retina display panels are noticeably more expensive.
The iMac is the worlds most popular all in one desktop computer and was last refreshed in May 2011 with quad-core Sandy Bridge processors and high-speed Thunderbolt ports
We are not expecting to much of a change in the next generation iMac but we are expecting a redesigned iMac and Mac Pro in 2013 after Apple CEO Tim Cook confirmed in June that Apple is working to release an amazing product for the desktop users is 2013