If you process personal data, you must make sure to protect the data. If you fail to protect the data, you can get a fine. How high it is depends on various factors. Reporting the breach timely to the authorities can help to have a lower fine.

But reporting it doesn’t make the fine go away. After all, you started to process personal data and are responsible for it. Alternatively, you could’ve opted for not processing personal data if you think you can’t protect the data adequately.

You can read all this here: https://gdpr-info.eu/issues/fines-penalties/