Assuming WLM stores that critical information you changed in the registry, a system restore (a standard Windows function) to yesterday (or whenever it last worked) might help.
But sometimes system restore fails for no clear reason, and it also might be that a user setting like this isn't stored in the registry, but on place that isn't included by Windows in making those backups.
To exclude the possibility that it is an issue with your email provider, I'd download and install Thunderbird (that's free and easy to uninstall again) and see if sending works from that program. Be sure you use WLM first to read all your incoming mail. If you don't, that ends up in Thunderbird also. Just use it to send a test message to your self.
By the way, you didn't tell WHY you can't send mail any more. Can you tell more about what happens, such as an error message?