Most likely the boot string - the location written into PRAM (or NVRAM, rather, on that machine) of where to begin the boot process has become corrupted for some reason. Zapping the pram might fix that, but it would be easier to go into Startup disk. Click to select no drives at all (meaning that it's being figured out at boot) and close. Open it again, select your start-up drive and close. This rewrites the bootstring again.