1. Once you know how your website was compromised, make sure that the vulnerability or weakness hackers used to get in is properly patched. Also, make sure that you have followed all of my earlier recommendations. Only after this step is completed should you run your website online again, otherwise you risk facing a second compromise.
2. If your customersí personal data was compromised, notify them and ask them to change all of their passwords as soon as possible. Assure them that you are taking the incident very seriously, an investigation is in progress and that you will do your best to ensure that it will never happen again. It is much better for your reputation (and your customersí security) to honestly notify them about the incident rather than trying to cover it up. But donít make a public show from the incident: sometimes thatís exactly what the hackers want to do to harm your business reputation. In many cases, a personal notification to each concerned customer is enough. There is no need to send a massive notification to everyone if only a couple of customersí accounts were compromised (just make sure that you are not mistaken about the scope of the incident!).
3. Depending on what legislation your country has about cybercrime, you may wish to deposit a criminal complaint against the attackers even if they are hidden behind a chain of proxy servers. Itís the job of law enforcement agencies and security companies specialized in digital forensics to identify and prosecute the hackers. However, donít be too optimistic as, due to a lack of inter-government collaboration and different laws in almost every country, many of these crimes remain forever unsolved. Nevertheless, it may bring results and at the very least will demonstrate to your customers that you take their security seriously.