Try repairing it with Disk Utility. It's in /Applications/Utilities/Disk Utility. Also, check the SMART status for the drive as reported by DU.
If DU isn't able to fix the problem, and the SMART status is ok, then invest in DiskWarrior. If the SMART status isn't ok, then it's time to get a new hard drive, regardless of if DU can fix it.