If you lost your Wikia password:
Go to this page: http://answers.wikia.com/wiki/Special:UserLogin
Put in your username
click the "e-mail new password" button
If you lost your password for another site:
See if the site has a way of retrieving passwords. There is often a "forgotten password" link on the login screen that you can click for info on how to do this. If there isn't, look for a way of contacting the website to ask them about it.