Facebook reviews all ads before they are allowed to run (this can take up to 24 hours).
This is why it is important to follow Facebook's ad policies.
If you have been notified that your ad was rejected by the Facebook moderators you can either:
Edit the Image: Using the information in your disapproval email, you can edit your ad and create a compliant one. Check this page for editing steps with Facebook.
Appeal the Decision: If you feel it was a mistake that your ad wasn't approved, you can appeal the decision with Facebook using this form.