14 DAY TRIAL // A lot of hacks happen these days, databases are being breached, sensitive info stolen and sometimes it's not even the database owner's fault. You can't have both data (being easy to access and to use) and tight security. There is no such thing as the perfect system, but there are some of them really hard to get into. Of course the companies from which the data is stolen are blamed, even if the hacker really had great skill. After such an unpleasant event there are not many things that you can do to come back in style.
So, data has been breached; what do you do? Well, first thing you should do is notice everyone that might be affected. If you have a database of 100.000 entries the best thing to do isn't to waste time to see which data has been stolen and which has been untouched. Notice everyone that they MIGHT be affected. You can do the search later and inform people whose data was stolen by the hackers. Of course, don't just tell them, but apologize as well.
Respond fast to the accident. Patch up those holes in the system and make sure that this kind of thing doesn't happen again. Another good thing is to quickly deploy a hotline service, or if that is not possible, then just make sure people get their questions answered. You may create a forum, post a FAQ on the site, or create a contact list of people that may answer questions. It is very important that people know what has actually happened, in order to properly act around it. Do not keep it a secret, otherwise you'll wake up with complaints and lawsuits.
Also, there is another important thing that you can do. It is ideal that you do this as soon as possible and not after the crisis, but it's better late than never - if you're sitting on a huge database and have a great network, make sure you have a good IT dept. And by that, I don't mean just to hire good people, rather keep in mind that 2 guys can't do the job of 20, as cyber-security is an on-going process.