All files have been erased after installation of Windows 2000

I have a desktop computer on which the Windows version is 98. I tried to install Windows 2000 on it. During installation, installation has been stopped automatically. I shut down the computer and launched the installation again and the installation has been stopped again.
Then I plugged out the cables of the hard drive and my brother installed Windows 2000. After installation, I saw all files on my hard drive have been erased. Why did it happen? Is there any procedure to get my files back?
