For this guide, I'm going to start with the quick step-by-step process how to resolve the most common keychain issues. At the bottom of this guide (for those interested in understanding why this happens), I will explain the common causes of these issues.

1. If you login to your computer and you see a window that looks like this  DO NOT click Continue Log In. This is will make your life miserable.
2. If you know your previous/old password, click Update Keychain Password. You will then enter your OLD password. This will update your keychain password to match your new password.
3. If you don't know your previous/old password, click Create New Keychain.

What do I do if I already clicked Continue Log In?

1. Click the Spotlight icon in the top right (magnifying glass icon) and enter Keychain Access.
2. Click the Keychain Access menu and select Preferences.
3. Click the  button.
4. Enter your login password.
5. Done!

I will now try to explain why this is happening.

Your Keychain is a secure place on your computer where it stores all of your saved passwords for websites and applications that you use. Every time you tell Safari to save a password for a website, it stores it in your keychain. The password used for iWeb to update your blog is saved in your keychain. When you enter your password into Mail for your email, it is saved in your keychain. 

Since your keychain stores so much important and private information, it can ONLY be accessed by entering your login password.

When you change your password with the CBE, it is updated in their servers, HOWEVER, it is does NOT update your keychain password. This means that when you change your password, the password used to login to your computer is different than the keychain login password.

So, in short, by either updating your keychain or creating a new keychain, it gets your login password and keychain password back in sync.

Hope that helps!