A simple rule of thumb is; You can revisit and continue working on your Whiteboard progress as long as your order is in a 'Draft' or 'Review' state. However, if you have proceeded through checkout and placed your order, it has moved to a 'Final Submission' state and, therefore locked.
To save your Whiteboard progress, simply click the 'Save' icon at the top right of the page. This creates a draft of your order based on what you have added to your cart.
While on the dashboard, select the associated order from the Orders Widget, when you're ready to continue working on your Whiteboard draft.