Yosemite won't recognize my (admin) user password
It will be hard to tell what has broken without being logged in, but the solution is easy.

Boot to the Recovery HD (power cycle the Mac, then hold Cmd-R until the screen goes grey. Then wait about a minute or so and you should get the recovery tools. See http://support.apple.com/kb/HT4718 for details) and reset the password to be what you want it to be. Once you get logged in again, you can look at the logs and patch up your Keychain as needed.