If you're having trouble sending mail, then either the SMTP (outbound) mail server settings — the server, the port, the user, the password, and whether SSL is in use or (unlikely) not — are incorrect, or the mail server is offline or has been reconfigured.
The SMTP (outbound) settings are in Preferences, but are buried rather deeply. Select Preferences > Accounts > select the account > switch the outgoing mail server (SMTP) to Edit SMTP Server List... and then select the SMTP server from the list, and verify that the settings are correct with your ISP, and that your username and password are correct.
The required settings vary by your particular mail ISP; you'll have to check with the ISP for the server and login details.
To test settings, select Window > Connection Doctor > then (when the tests fail) show the details, and (if you can't sort out what happened) sanitize the settings and post the output here.
Caveat: do not repeat password tests with the mail server, as various mail providers can be configured to assume these are a password brute-force attack, and lock out repeated login attempts.
Caveat: never, ever, ever, ever, ever, ever, ever not-at-all enter a password when Mail pops up a dialog box with the "OMG YOUR PASSWORD IS WRONG!!!!!!!!!!! PLEASE ENSURE YOU CANNOT LOG IN AGAIN!!!!!" dialog box. (I'm paraphrasing that error dialog box slightly.) The correct answer to that Mail.app password dialog box is almost always "cancel" button. Passwords stored in Keychain generally don't get lost or corrupted, and trying random passwords as that dialog box encourages just gets you locked out of various servers, and...