This means the firmware can't find the bootable volume it was assigned to it in System Preferences > Startup Disk.
It may be nothing, try this easy fix first. Press the power button down until the machine turns off if it's not off already.
1: Hold the option key down on a wired or built in keyboard and press the power button.
2: A bunch of icons appears, one SHOULD be your Mac OS X boot volume, click the arrow underneath to boot from it and then head to System Preferences > Startup Disk and select yours and apply.
That will remind the firmware what to boot from.
If you don't see that, then your going to have to perform data recovery if you don't have a back up of your user files yet
then attempt to fix