First, make sure that you are using the correct Facebook User ID and password. You can verify the User ID and password combination by attempting to log in at http://www.facebook.com.
Please do not use User IDs or passwords that are saved in your browser.
If you are able to log in by typing the same User ID and password that you used when attempting to import your contacts, then please attempt to import your contacts again. If you are still unable to import your contacts, please use the following link to Contact Customer Care.