my mac mini stoped using my log in pass how do i re set it

1. Insert the Mac OS X 10.4 Install disc, then restart the computer while holding the C key.

2. When your computer finishes starting up from the disc, choose Reset Password from the Installer menu at the top of the screen. (In Mac OS X 10.4 or later, you must select your language first.)

Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Reset Password.

Or...

Reset OS X Password Without an OS X CD...

http://theappleblog.com/2008/06/22/reset-os-x-password-without-an-os-x-cd/

Admin Hack...

http://www.hackmac.org/?q=node/4

Starts up like the first time you buy a new Mac, but after filling in all that info again, you should have access to the computer and the other Users & files will still be there... give the new User a different name than an existing one.

BDAqua has explained how to reset your password. However, it's important to understand that your computer won't just suddenly decide to stop accepting your normal password. If you are entering the correct password and it is no longer accepting it, one possibility is that someone has reset your password themselves, in order to snoop on your computer. If someone who might want to snoop on you has physical access to the computer, you should consider setting a firmware password:

http://support.apple.com/kb/HT1352

This will prevent them from resetting your password again without knowing the firmware password... just don't lose the firmware password! You should also encrypt any sensitive data, as there are ways a knowledgeable person could access it without resetting your password.

If you simply forgot your password, or are having other issues than just a suddenly and inexplicably rejected password, then that's probably not a sign of someone snooping.