Have you contacted your mail provider to check that their servers are up?
Does the mailbox work on other devices?
Have you verified that you have entered the correct account information in Mail/Settings/Accounts?
You need to make sure your account name, email address and password are correct.
If that doesn't fix it then you need to confirm the other settings (IMAP and SMTP servers, ports, authentication) with your mail provider.
If all the above fails and you are certain that you know your account details then delete the account and create it again.