Boot into single-user mode. After startup is completed you will be in command line mode and should see a prompt with a cursor positioned after it. At the prompt enter the following then press RETURN:
If you receive a message that says "***** FILE SYSTEM WAS MODIFIED *****" then re-run the command until you receive a message that says "** The volume (name_of_volume) appears to be OK." If you re-run the command more than seven times and do not get the OK message, then the drive cannot be repaired this way. If you were successful then enter:
and press RETURN to restart the computer.
There is no guarantee this will fix the problem. Normally you must boot from another drive in order to repair the startup volume. If you can't use a Snow Leopard DVD then you would need to use some other bootable device like a backup drive or another Mac connected via Firewire - Target Disk Mode.
It looks like the filesystem (not the hard drive) is damaged. Fortunately, there's no risk because your hard drive isn't damaged, so you just have to get a Mac OS X DVD, and you need it.
If you can't access to a Mac OS X DVD, you can still repair the disk. First, press Command and S keys while your Mac is starting to start into single-mode user, without a graphic interface. Then, after starting in this mode, type: fsck -fy
As you have root privileges in that mode, you won't have to insert the password. When "fsck" finishes, type "exit" (without quotes) and your Mac will start as normal. To see if the volume was repaired, open Disk Utility, select Macintosh HD in the sidebar and verify the drive
I will try this, thank u so much. should i bach up my hard drive before?
Yes, you should be backing up all the time as the machine can fail at any time.
Use the emergency quick copy method here
Also your signature says 10.6.8, but if your on 10.7 or 10.8, then simply hold the command r keys down while booting the machine and use the Disk Utility there to repair the Macinstosh HD partition. (not the 10.6 install disk)
If you have Internet Recovery, then use command option r upon boot and repair the entire drive.