If your message is stuck on S: The person you're messaging isn't connected to Kik (maybe their phone is off). Your message will be delivered once they turn their phone on or sign into their account and reconnect to Kik. You or the person you're messaging is experiencing some connection problems.
How do you know if you have been muted on Kik?
You cannot tell if you've been muted on Kik. This is a discreet way of someone shutting someone up that's messaging them a lot or someone they do not wish to message at the current time. Your message is delivered as normal, they just don't get a push notification on their device that you messaged them.