Client thing keeps saying "connection lost" in a single player world and will not close!

Hi, I'm new to the forum so I'm not sure if i chose the right forum section, But as title says, When i go to test my mod out, It plays normally for a while in a single player world, but then it says connection lost after a few minutes and then takes me to the multiplayer tab. I can't close or stop the client afterwards either. How can i fix this?