According to the source above they sent emails to all there customers to change the email/password on priority bases.
In the case of credit card information, they have said that their banking information is all safe and stored on a totally different server.
To change your Plex account password, open a private/incognito browser window. Next, enter your email address and hit “Send Instructions”.
Within a minute or so, you should receive an e-mail that will give you instructions.After changing your password you can either stay or delete your account safely from the plex network.