While it is difficult to know what may have happened on your installation without more specific details, you may want to boot your computer from an external drive or use it into FireWire Target Disk Mode -- this is not a requirement but will make the following step easier.
Then, locate a file called "[username].sparseimage" -- the one we are talking about in the article. While booted from the hard drive, it will be normally hidden but can nevertheless be located easily.
Finally, try double-clicking on its icon to open it, like any other password-protected disk image.
Let me know if this helps !