Could be lots of reasons for that.
Can be the wrong mail server settings, or can be that your mailbox needs to enable access for third party mail clients or could be you have to manually setup your account if it’s a non OAuth legacy login depending on your specific server.
So first thing, make sure you can login to your mailbox via your webmail in a browser. As that can also happen if your mail account has been deactivated.
Now if you can login ok to webmail with your same login and password, then check with your mailbox technical support via website or phone if you need to enable third party mail client access as some mail boxes do.
Now if you don’t need to enable third party mail client access or you do but made no difference, then could be you just have the incorrect mail server settings in eM Client.
So update your “Server address, Port and Security policy” via “Menu / Accounts” via the tabs along the top . Also update if it’s a POP, IMAP, Exchange, Office 365 or iCloud account and who hosts your email so we check if there is anything obviously incorrect.