Did you receive an email with the subject 'Forgot password'? Then you probably clicked on the 'forgot password' button in the log-in screen of your Mollie dashboard (see picture).
When you didn't do this it is important to check whether other people have access to your account. Have you ever given your log-in data to a colleague or your site builder? Then approach them to find out whether someone has used the password reset feature.
And, better be safe than sorry: set a new password. You can do this on your dashboard via Account.