Check the access to the mailbox through POP3 / IMAP / SMTP protocols. Sometimes this option can automatically switch off. If you (or anyone else) changed your mailbox password then it needs to be restored also in e-mail program.
! Make sure you enter the correct password - try to enter the mailbox through the web home page. If you can get new messages but you cannot send them then the error is in a SMTP settings. Perhaps it is necessary to turn on authorization regime to outgoing SMTP server.
! ! If you can send messages, but cannot get them then the problem is on POP3 or IMAP settings. If you are are not able to independently solve the problem, contact us your question, add a screen image where is visible incoming and outgoing server settings of your mail program, and specify the mailbox address, which encountered these problems.
In case you did not find your answer contact us