Inspect the top of the drywall. If there is anything black on it, get it replaced. It already has grown mold and you don't need that in a new home. If it's only water stained, then dry it and seal it with the sealers that were mentioned above. For the most part you can get away with a one time leak and not have an issue, but you really don't know how long this may have ben wet before it broke through.