After change the file access option 'No Access', I cannot start up my iMac. Only blue screen!

Question

Hi experts,
I really need help.
I have several user accounts in my iMac G5.
Some reason - I don't know why I did - I went into the 'get info' of the Mac hard drive, and change the permission to the hard drive from 'Read and Write' 'to 'No Access' for the one of the user accounts.
After that, I switched the user as the one I just changed 'No Access'. But I couldn't do any action in that user space, like opening finder, starting any program.
So I re-started the computer and since then, I only see the blue screen (no arrow no nothing.).
I tried booting with Safe Mode, but still only show blue screen.
I tried booting with single user mode, still blue screen.
I can start up with the disk but couldn't restore the HD since there is no time point at all that I can go back.
What should I do? Please help me~

Answer 1

I tried booting with single user mode, still blue screen.
Changing permissions an a user account should not cause single user mode to fail.  You changed permissions on something else. Could you provide more info on the folder you changed?
Some reason - I don't know why I did - I went into the 'get info' of the Mac hard drive, and change the permission to the hard drive from 'Read and Write' 'to 'No Access' for the one of the user accounts.
This is a little confusing.  We need to know what folder you changed.
You might try Disk Utility repair permissions. Here is how to get into Disk Utility from the startup disk.
You can also get into the Terminal. Notice terminal is on the pull down. Check permissions on the root directory
mac $ ls -ld /
drwxrwxr-t   51 root  admin    1K Dec 31  1969 /
mac $
verify & repair your startup drive
To verify & repair you file system on the startup drive, you will need to run disk utility from you installation DVD.
This article  will tell you how to get to disk utility.  Once in a disk utility, you can go and attempt to recover the disk.
http://support.apple.com/kb/TS1417
To repair your startup drive, you will need to run disk utility from your startup DVD.
Mac OS X 10.4: About the utilities available on the Mac OS X 10.4 Install DVD
http://support.apple.com/kb/HT2055
How to run disk utility from your startup DVD.
Insert your  startup DVD  into your reader.  Power down your machine.  Hold down to the c key.  Power on your machine.  This will bootup your startup DVD.
This will bring you to a panel asking you for your language.  Pick your language.
You you come to the Install Mac OS panel.  Do not install.
Click on Utilities menu item.  This will give you a pulldown list of utilities.
Click on the disk utility.
You are now in disk utility.  Pick your disk.  Click on repair it should be on the lower right of the panel.
Once the repair completes successfully, you should update your permissions.
Verify a disk
As an alternative, you can verify that the filesystem on the disk is correct. You will not be able to repair the file system.
I suggest that you use disk utility to verify that your startup disk is OK. 
Macintosh-HD -> Applications -> Utilities -> Disk Utility 
Start up disk utility.  On the left pane view, you will see a list of all your disks.  
Click on your startup disk.
Click on the First Aid  Tab.
Click on verify.  
Hopefully your disk will verify.  If not, you have to boot from your installation DVD and run Disk Utility from there to attempt to repair your file-system.