Locked Out of My 15" Macbook Pro

Hi, I locked myself out of my 15" macbook pro. Is there a keyboard code that will give me access to bypass the blank screen and allow me to change settings? I can't get to my password login when I start up the computer.

As far as I remember, I changed a security/privacy setting. I think I right clicked the HD symbol, then clicked preferences (get info.) I found a users and ownership/permissions list. This is where I clicked a drop-down arrow menu "owner" and "you can" and then within that sub-menu I denied myself permission to read or write to the hardrive. Immediately following this action, the computer shutdown and went black. I restarted the computer. There was no longer a start-up screen for password entry. I can see the typical Macbook apple "blue" screen which is behind password login but the password entry boxes are gone. This shouldn't be confused with what I've heard is called "the blue screen of death" because this is just the default macbook pro desktop background. Thank you for any help you may offer!

Also, I'm not absolutely sure if the operating system is Mac OS X 10.5

Thanks again,

-Beau