Who is your host?
Double check in Universal Email, that you are using a valid email address for your domain as the from address.
And double check the SMTP sever and port you are using, if you are not sure of the correct server or port, contact the host.
Where you say it was working up to today, it is possible that the mail server has a high load and is taking some time to really the email, I have seen instances with Go Daddy where emails could take as long as a day to send through the server.
Do you have Universal Email set up to go to another page after sending?
Is it going to that page?
try adding the following code to go to page to dump the session contents:
<?php @session_start(); var_dump($_SESSION); ?>
the session contents will include the status of relaying the email to the SMTP server. if that says success, then you need to check with the host to determine why the SMTP server is not relaying the email.