reboot your computer and run fsck by pressing command (the apple thing)+s to start up in single user mode, then type in "/sbin/fsck -y" don't forget to leave a space between fsck and -y and just wait.
If it tells you "(name of hard drive) seems to be OK" then type in "reboot". If it doesnt and says it needed to modify some files run fsck again until it tells you it's okay, then reboot.