Are you seeing this?
Firmware is telling you that it can't find the bootable operating system it's been assigned to boot from.
Machine is powered on? Hold the power button down to force a emergency shutdown. (do not use regularly, use Shutdown from Apple menu)
Have a wireless keyboard? You will need a built in one or wired one to get any further. A PC one will work.
Hold the option/alt key down and power on the computer to enter Startup Manager.
If your OS X boot volume appears (usually Macintosh HD unless it was changed), simply select it and click the arrow to boot from it, then head to System Preferences > Startup Disk and set it again.
Reboot normally to test. Finished.
If this doesn't solve the problem, then continue.
OS X 10.4/10.5 only
Those versions of OS X are obsolete and you need to consider upgrading if you have a Intel Mac. (can't if you have PPC Mac)
OS X 10.6 and up
More steps will have to be performed to determine what's going on, data recover performed an/or OS X erased and installed.