How do I fix Roadrunner is suddenly not sending or receiving emails?

My RoadRunner email account is not downloading or sending messages. I can log in and have tried the sync button, but nothing happens. Has anyone else experienced this issue? Any suggestions for resolving it would be appreciated!

In order for someone to be of assistance, please provide more information about your installation. Make and model of device? Android version (or chromeOS in the rare case if you happen to be using a Chromebook)? Version of K-9 Mail? POP3 or IMAP? Account settings? The more information provided, the more likely someone will be able to assist. Thanks.