If your campaign is in pending or approved status, you will see an option to stop the campaign. Once you stop a campaign, you cannot restart it. You will have to create a new campaign.
Within 48 hours, the campaign will show your remaining credit balance and you can use these funds towards a new campaign. If you do not use that credit towards another campaign within 90 days, a refund will be issued to you automatically.
If you would like to stop your campaign due to performance reasons (ex. Ad is not getting as many impressions as you'd like), we highly recommend waiting until 24 hours after your ad start date. While ad reporting is near real-time, some results can be delayed. Waiting 24 hours will allow you a full picture of how your ad is performing.