Welcome to Apple Support Communities
Turn off the computer holding the Power key. Then, turn it on and hold the Option (Alt) key until you see all volumes on the display. After that, choose your OS X partition. Finally, open System Preferences > Startup Disk, and choose the OS X volume.
That happens when your computer has changed its startup disk to one that isn't available now. If you can't start up holding the Option key, reinstall Mac OS X