hi
i accidentally gave 'no access' to my primary drive where the OS is installed in my G5, and then everything went blank. cant access any drives.

Wow, I never ran into that one! Can you describe exactly WHAT you did to cause this problem?

I think you will need a way to boot from another disk, such as an external firewire drive so that you can undo the damage. Are you located anywhere near an Apple Store? If so you should be able to bring the machine in and have them help. Another possibility would be connecting your mac to another one and using firewire target disk mode to mount its hard drive. Here's a description of how to do this (the "first computer" would be the damaged one):

1. Shut down the first computer and leave the second computer on.
Connect the two computers using a 6-pin to 6-pin FireWire cable. (If both computers have higher-speed FireWire 800 ports in addition to the standard FireWire 400 ports, you can use a 9-pin to 9-pin cable with the FireWire 800 ports to transfer data at higher speeds.)

2. Start up the first computer while holding down the T key.
A disk icon for the first computer appears on the desktop on the second computer.

BOOT TO THE ORIGINAL INSTALL DVD!!!!!!!

CHANGE THE ADMIN (ROOT) PASSWORD

That should (I think) reset all settings on the drive, or at least let you get to it.

As Tomas suggested boot from the OS X install disk. When you get to the first install screen, select "Disk Utility" from the Apple menu and then "repair permissions." This should work.