ThinnerNana,
No, you do not need to enter a new registration code.
You will be asked to restart your account if 1) you have met your weight-loss goal and would like to set a new one, or 2) you have missed more than 4 check-ins and are now 'inactive'. By restarting, you can enter a new starting weight and goal weight, start dates, and you can select any step to restart on, up to the last step you received.
You will still have access to all the tools, and the message board. There are some great threads and awesome people who might be able to help you through your tough time and inspire you to jump back on the bandwagon.
Best wishes,
alliadmin3