There may be various reasons for an inability to access your ProtonMail account, including wrong username/password, account bans, or other issues related to the browser. To begin with, make sure that your username and password have been entered correctly. Remember that ProtonMail passwords are case-sensitive. Are there any login errors or typos? Maybe the caps lock is turned on? If you do not remember your login password, kindly click on the black link ‘Forgot Password?’ located on the login window. When closing your account, do follow the link ‘Delete Account,’ as resetting the password on ProtonMail will not retrieve old mails because of their encryption.

However, if you have checked your password and username and they are both correct yet you cannot access your account, have a look at your recent activity notifications for messages about account bans. Cases like this can happen due to breaches of security or infringement of any ProtonMail terms. In such situations, go to the ProtonMail help page and contact their support.

Lastly, browser-related problems can act as a blockade to successful login attempts. Try deleting the cache and cookies of the browser or use a different web browser or device. Alternatively, if the problem remains unresolved, look up other issues on the ProtonMail website or ask for help from ProtonMail Customer Support. They are competent in such matters and will do their best to make sure you regain access to your account.

More Info: https://www.resetprotonpassword.com/how-to-reset-my-password-for-protonmail/