Check that you have entered your number and password correctly - password fields are case sensitive - so PassWord is not the same as password, make sure you have the right one.
If your mobile number has been barred you will not be able to login to your account online.
If your mobile number has been disconnected you will not be able to login to your old account online.
If you are in arrears with your account payments, you may not be able to login online.
If none of these applies, and you still can't log in, then please contact customer services.