Hold the option key down while booting the machine (wired keyboard) and a list of bootable options appears.
Select OS X and click the arrow, then set the Startup Disk again in System Preferences. Hopefully this will fix the issue and no further problems.
If no bootable options appears, then your internal boot drive could be dead.
If you have 10.7 or later and doing the above shows only RecoveryHD, then your Macintosh HD partition is corrupted.