I was logging on to hypixel. When i heard. Failed to authenticate your connection! I went to log on to mineplex and it said the same thing but with minecraft.net. If you can tell me how to fix this please do!

How to fix?

If your username has changed and didn't notice. Relog into your launcher. The reason why is because your server you tried to log on to now has your name data for your new USERNAME. Thank you for all the help guys!