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
A Quick Guide on Setting Up The Default OS in BootCamp When using Windows
BootCamp is a control panel that lets you know and decide on what OS your Mac starts in. To select the default operating system, you need to follow the mentioned steps.
• When in Windows, click on the Boot Camp system tray and go to the BootCamp Control Panel.
• You will see the different drives to select from. Select the drive which has the OS of your choice. This will become the default preference for your Mac.
• If you want to see whether the preferences have been accepted or not, Restart to check the changes.
Not just this, BootCamp also lets you start your Mac in Target Disk mode. This mode lets you use your Mac as an external FireWire disk. To read more about this and see what all you can do, try restarting your Mac using Mac OS X and choosing Mac Help from the Help Menu.
To set up the default OS when running Mac OS X, try the Startup Disk pane under System Preferences.