Have I already been hacked?
Has a company holding my personal data already been hacked?
I am going to introduce two websites here that let you see whether some of your personal data is publicly accessible, and therefore has been stolen.
The Have I Been Pwned site: haveibeenpwned.com
Its name comes from English and would translate into French as Ai-je déjà été piraté ?

It is a site maintained by a Microsoft engineer that lists all the public databases collected on the Internet and accessible by anyone.
Checking from an email address
You can query this database from the home page by entering your email address. This will check whether there is an entry concerning you.
You then need to click "pwned?" to start the search.
If you are told that you have indeed been linked to a hack, you can then scroll down the page to see more details about the source of the disclosure.
Here is an example of two breaches where personal data was disclosed.

Checking from a password
In the "passwords" section you will find the option to test your passwords .

This feature is very useful because it lets you check the strength of your password in addition to checking whether it has already been disclosed.
If your password has indeed been found in one of the databases that was hacked, you can again scroll down the page to find the list of files containing the password, along with all those disclosed with it. This list can be downloaded.

It is now important to remember that if your email address or your password has been disclosed, you must take measures such as changing and strengthening the password, or enabling two-factor authentication if necessary.
