It sounds like you have changed your email address or password on this website. Try going into your settings here and see what the email address is set to and make sure that this is what you are using in the app. You can also double check your password too.
I have this problem too. I'm using iPod, and the app stopped syncing. So I deleted the app, updated it on my Macbook, and then reinstalled it. The app now won't accept my email/password combination. I logged out on the website, and went through the 'forgot password' options to login with a new password. The new password gets me onto the website, but isn't accepted by the app. Lots of rebooting the iPod has so far achieved nothing.