If one of your clients cancels their SEO plan for some reason, you are supposed to ask them to specify whatsoever their issue of withdrawal is. Make sure to rectify any mistakes if the reason is from your side. Help them find a solution to their problem that caused them to cancel the plan. Try to make the client stay if possible, with negotiations. However, if the client persists to leave, let him cancel the plan and move on to focus on other potential clients.