Your project is saved inside your account so in order to save and resume a project you will first need to create an account. If you need help with doing so just follow the instructions founds in this link:
Once your account is created and you have started a project you can always resume your last saved status by clicking on "my projects" located in the top right menu of the site. This is how it looks:
Once you've clicked the button you will be able to see your current projects as well as past projects that have been completed and ordered.