Despite its unique features and functions, sometimes it is difficult to log in. If you find that the Protonmail account temporarily unavailable, then this is because there is anything that looks like an attempt by an attacker to access your account. Protonmail's security system will automatically identify and flag it. When you are at high risk, Protonmail may also temporarily lock your account to prevent the attacker from accessing it. Further, a corrupt browser cache is often the cause of this problem. Therefore, you should clear the browser's cache and restart the browser. Also, you may need to change your domain if you want Protonmail to accept your account else it will restrict users from logging into their accounts.
See also -
How to Change ProtonMail Password
Why is my ProtonMail Not Receiving Emails
Why is my ProtonMail Not Working Today