How do I stop my all-day parking session ? and why did the timer disappear ?

When you initiate an all day parking session it only needs to show you that it started. This is not an error. It happens because an all day parking session does not need to be terminated. It needs only to be initiated. All day parking sessions automatically end at the end of paid parking hours each day. The system will also not allow you to pay for the same all-day zone twice in a given day, so if you do try to initiate it again - it will simply tell you that it already started.
If you are confused and are not sure that you have initiated a parking session you can press the 'Start' button again – but check that the correct zone is selected. If you receive a message “parking already started” then you are OK. You will not be charged twice – it just means that the session is in progress. If you did forget to start the session - then it will simply start.