There are two possible reasons why you might not be able to log in to your AdSense account.
1. You're using your Google Account password to sign in to your AdSense account.
2. Your Google Account password and AdSense password are the same, creating a login conflict.
To resolve the issue, please follow the steps below.
1. First, clear your cache and cookies by following the directions at
http://www.google.com/support/adsense/bin/answer.py?answer=23767
2. Next, visit
https://www.google.com/adsense/assistlogin . On this page, you can submit your email address to us. We'll then automatically send you an email with a link that will allow you to reset your password.
3. Once you receive the email to reset your password, please choose a new password that you're not using with another Google product.