If you cannot log in to Webmail, connect from Mail client/plugin or send emails, the first thing you need to check is Mail services. You can find them at Hosting → Manage → Mail Service Control:
If mail services are disabled, the mentioned behaviour is expected. No worries, as you can enable mail services again in a few clicks 😊
Mail services can be disabled automatically if one of the conditions is met, please check them both and how to solve the issue permanently in this article.