Whenever you need to log into your Proton account, follow the instructions in the email. First, you will need to enter your emergency passcode, which you can copy to your smartphone. You can also use this emergency passcode to get access to your account even if your phone goes kaput. Secondly, you can check the list of the various sessions you have open. If there is an existing session, you can shut it down.
See also – https://www.nativesnewsonline.com/protonmail-email-account-and-other-login-information/