If the password box is filled automatically this means your web browser has stored a previous entry you made in a similar password box. The numbers/letters do not come from us and may not be your latest password.
Check the entry for errors. In the password entry box is an eye symbol you can click to see your password as you enter it. This will remove the dots that usually mask the password. You can then check the password has been entered accurately.
If you are using copy and paste ensure there aren’t any unwanted blank spaces before or after the password.
If this doesn’t solve the problem you can use the “Lost your password?” link to reset your password.