Click the 'Forgot your password' link just above the green Sign in button.
Enter your email address (ensure you spell it correctly) and click the "Send Password Reset" link.
You will then receive an email that contains a link to change your password. Open up the email and click on the 'Change Password' link in the email so you can choose a new password.
If you are still having issues with signing in after trying this, please contact our help desk here.