We often have visitors who are very certain that they have the correct password, but we end up finding that they have not noticed that there is a special character at the end of their password such as a period or exclamation point.
Then be sure you are typing it in, not relying on information that may be saved in your browser. If there is already something in the password field, delete it out and re-enter it. Be sure that you are capitalizing the password exactly like you originally entered it. You may need to clear your browser's cache buffer.
You can also check the box to Show Password which shows what is being submitted.